Another reason for sudden fall is IL&FS debt crisis . It has defaulted over 3 payments this month . Since alot of banks have exposure to it's 90,000 crore debt, banking stocks are also seeing a sell off owing that they may to write off this amount . Although LIC has come to it's rescue by buy agreeing to subscribe to it's right issue ,but only the time would tell whether they will able to save this crisis or not.

The crash followed an asset bubble. Since 1922, the stock market had gone up by almost 20 percent a year. Everyone invested, thanks to a financial invention called buying "on margin." It allowed people to borrow money from their broker to buy stocks. They only needed to put down 10-20 percent. Investing this way contributed to the irrational exuberance of the Roaring Twenties.
Nintendo reserved a large part of NES game revenue for itself by limiting most third-party publishers to only five games per year on its systems (some companies tried to get around this by creating additional company labels like Konami's Ultra Games label); Nintendo would ultimately drop this rule by 1993 with the release of the Super Nintendo Entertainment System.[50] It also required all cartridges to be manufactured by Nintendo, and to be paid for in full before they were manufactured. Cartridges could not be returned to Nintendo, so publishers assumed all the financial risk of selling all units ordered. As a result, some publishers lost more money due to distress sales of remaining inventory at the end of the NES era than they ever earned in profits from sales of the games.
Soaring home prices, combined with 50-year low interest rates, have lulled U.S. homebuyers into a false sense of security. But current economic conditions, combined with the actions of overly aggressive lenders, leave the housing market ripe for a major crash. The Coming Crash in the Housing Market is the first rational, unbiased examination of the dangers homeowners face in today's climate of overpriced housing and overextended credit. Asking and answering questions that have for too long been ignored, respected economic consultant John Talbott provides:
According to Lee, there are two key factors that will soon bring more institutional interest to the markets. First, it will be the upcoming launch of the digital assets platform Bakkt by the operator of major global exchange New York Stock Exchange (NYSE), Intercontinental Exchange (ICE). Announced in August this year, Bakkt recently confirmed a “target” launch date for Jan. 24, 2019.
For the rest of the 1930s, beginning on March 15, 1933, the Dow began to slowly regain the ground it had lost during the 1929 crash and the three years following it. The largest percentage increases of the Dow Jones occurred during the early and mid-1930s. In late 1937, there was a sharp dip in the stock market, but prices held well above the 1932 lows. The market would not return to the peak closing of September 3, 1929, until November 23, 1954.[17][18]
These stocks are known as high beta stocks, as they outperform on the way up and underperform on the way down. During a bull market, these high beta stocks are often the stocks that perform best. As a result they will grow into the largest positions in your portfolio. That’s why it’s a good idea to rebalance your portfolio and make sure the weighting of these “high beta” stocks aren’t too high. Here some more ways to prepare for a stock market crash:

Indian stock markets witnessed a steep and sudden sell-off in the afternoon deals on Friday. Rajat Sharma of Sana Securities told FE Online that stock markets are "extremely overvalued" and Sensex can fall even 2,000 points from here while NSE Nifty can correct by about 1,000 points. "Nothing has changed fundamentally, I mean we have the same macro-economic situation, etc, but when a sell-off happens, nobody can predict, Rajat Sharma said further. 
While the note's warnings are ominous and contradict many other more rosy outlooks for the current bull market, the London-based fund was on point in calling February's market correction weeks before it happened. Filia told CNBC in late January that stock valuations were in "bitcoin territory," "totally disconnected from fundamentals," and that markets were on the "edge of chaos."
The cost of ownership in the most high priced markets is going up even more. Why? The limitation on the mortgage interest deduction to $750K and the limitation on the sales and property tax deduction to $10K. With the increase in interest rates, the partial non deductibility of interest and taxes, the overall cost of ownership is going up. Most people will feel the punch in their guts next year when they file the 2018 taxes. That is when most folks would realize what hit them was not a pleasant surprise!
Other scientists disagree with this notion, and note that market crashes are indeed “special.” Professor Didier Sornette, for example, a physicist at the Swiss Federal Institute of Technology, argued that a market crash is not simply a scaled-up version of a normal down day but a true outlier to market behavior. In fact, he claims that ahead of critical points the market starts giving off some clues. His work focuses on interpreting these clues and identify when a bubble may be forming and, crucially, when it ends.
Stock market downturn of 2002 9 Oct 2002 Downturn in stock prices during 2002 in stock exchanges across the United States, Canada, Asia, and Europe. After recovering from lows reached following the September 11 attacks, indices slid steadily starting in March 2002, with dramatic declines in July and September leading to lows last reached in 1997 and 1998. See stock market downturn of 2002.
The Indian rupee strengthened further against US dollar in the early afternoon deals on Friday following the sustained weakness in the crude oil prices. The domestic currency (rupee) extended morning gains on Friday and hit a fresh 2-week high at 71.7663, up 62 paise per unit US dollar, the Bloomberg data showed. The rupee is trading 120 paise higher from the all-time low of 72.97 apiece US dollar. Earlier on Tuesday this week, the rupee went very close to hitting 73/$ and made a record low at 72.9675 against US dollar. 
On October 19th 1987, $500 billion in market capitalization was evaporated from the Dow Jones stock index. Markets in nearly every country around the world plunged in a similar fashion. When individual investors heard that a massive stock market crash was occurring, they rushed to call their brokers to sell their stocks. This was unsuccessful because each broker had many clients. Many people lost millions of dollars instantly. There are stories of some unstable individuals who had lost large amounts of money who went to their broker’s office with a gun and started shooting. A few brokers were killed despite the fact that they had no control over the market action. The majority of investors who were selling did not even know why they were selling except for the fact that “everyone else was selling.” This emotionally-charged behavior is one of the main reasons that the stock market crashed so dramatically. After the October 19th plunge, many futures and stock exchanges were shut down for a day.
Evan of My Journey to Millions took the conversation back to the bigger picture with your investing goals and, “I honestly do not think you can protect against a stock market crash, and that’s okay! Make sure your risk tolerance matches your asset allocation and ride it out knowing that you should have time to let it all work itself out.  It is unlikely that the next crash is going to be the one that destroys our market system.”
There are other mitigating factors too such as the strengths in the economy, foreign investors buying property, and rising optimism and confidence since Donald Trump won the election.  At this point, we’re wondering if Obama and Clinton are relieved not to have to face the mess they created? Trump seems to be up to the task and yet, he has purportedly said he would enjoy watching the crash, even if it takes down some of his real estate empire. Is this just a comment on high home prices?
The North American video game crash had two long-lasting results. The first result was that dominance in the home console market shifted from the United States to Japan. By 1986, three years after its introduction, 6.5 million Japanese homes—19% of the population—owned a Family Computer, and Nintendo began exporting it to the U.S.;[44] by 1987 the Nintendo Entertainment System was very popular in North America.[48] When the U.S. video game market recovered in the late 1980s the NES was by far the dominant console, leaving only a fraction of the market to a resurgent Atari. By 1989, home video game sales in the United States had reached $5 billion, surpassing the 1982 peak of $3 billion during the previous generation. A large majority of the market was controlled by Nintendo; it sold more than 35 million units in the United States, exceeding the sales of other consoles and personal computers by a considerable margin.[49] Other Japanese companies also rivaled Nintendo's success in the United States, with Sega's Mega Drive/Genesis in 1989 and NEC's PC Engine/TurboGrafx 16 released the same year.
Some of these motivations come from people all following each other, trying to predict the exact economic actions of other people all engaged in the same activity. (People who bought a stock at too high a price are looking for greater fools to unload it on.) While the market's open, everyone's trying to figure out the optimal value for the price of every stock everywhere. It's exhausting to think about the trillion or so variables that go into that immense labor of capitalism. It's crazy to consider how complicated the chains of cause and effect and overthinking are.
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In April 2015, Navinder Singh Sarao, a London-based point-and-click trader,[62] was arrested for his alleged role in the flash crash. According to criminal charges brought by the United States Department of Justice, Sarao allegedly used an automated program to generate large sell orders, pushing down prices, which he then cancelled to buy at the lower market prices. The Commodity Futures Trading Commission filed civil charges against Sarao.[63][64] In August 2015, Sarao was released on a £50,000 bail with a full extradition hearing scheduled for September with the US Department of Justice. Sarao and his company, Nav Sarao Futures Limited, allegedly made more than $40 million in profit from trading during the Flash Crash.[65]
The Indian rupee jumped as much as 55 paise against the US dollar in the opening trade at the interbank foreign exchange market on Friday. The rupee regained a level of 71.8288, up 55 paise per unit US dollar, the Bloomberg data showed. India's government is planning to ask state oil firms to lock in their crude futures purchase prices, Reuters reported citing an unidentified government source, anticipating a spike when US sanctions on Iran snap back again in November. The move would be another step to tackle a slide in the rupee, as oil prices are putting pressure on India, the report added. 
Throughout his presidency, questions arose from his handling of various events, including one self-inflicted crisis after another. Tensions rose as he fired Michael Flynn and then FBI director James Comey. The selloff on the morning of May 17, 2017 occurred after reports that Comey was asked to drop the formal investigation into Flynn. If these allegations are true, this could represent the same sort of obstruction of justice which lead to the impeachment calls and, ultimately, resignation of President Richard Nixon.

The second reason is that it is impossible to predict the beginning of a bull market. By sitting through the crash, you are basically ensuring that your investments are safe and rolling. History teaches us that stocks rally back to their old levels, given some time. Also, stock crashes in the last 100 years have lasted an average of just over ten months. So if waiting is an option, it would be the best one.