The Panic of 1987
Index arbitrage and portfolio insurance (programmed trading) were the major cause. From Tuesday 10/13/87 to 10/19/87, the DJIA fell 769 points or 31%. On 10/19/87 the DJIA fell508 points or 22.6%. On 10/28/29 the DJIA fell 11.7%.
Mutual funds and pension funds use portfolio insurance. Portfolio insurance is a strategy that uses computer based models to determine an optimal stock/cash ratio at various market prices. Two insurance users called for sales equaling 50% in response to a 10% decline in the S&P 500 Index.