Possible causes for the decline included program trading,
overvaluation, illiquidity, and market psychology.
A popular explanation for the 1987 crash was selling by program traders, most notably as a reaction to the computerized selling required by portfolio insurance hedges.[10] However, economist Dean Furbush points out that the biggest price drops occurred when trading volume was light.[11] In program trading, computers perform rapid stock executions based on external inputs, such as the price of related securities. Common strategies implemented by program trading involve an attempt to engage in arbitrage and
portfolio insurance strategies. As computer technology became more available, the use of program trading grew dramatically within Wall Street firms. After the crash, many blamed program trading strategies for blindly selling stocks as markets fell, exacerbating the decline. Some economists theorized the speculative boom leading up to October was caused by program trading, and that the crash was merely a return to normalcy. Either way, program trading ended up taking the majority of the blame in the public eye for the 1987 stock market crash. U.S. Congressman Edward J. Markey, who had been warning about the possibility of a crash, stated that "Program trading was the principal cause."[12]
New York University's Anna Chen divides the causes into macroeconomic and internal reasons. Macroeconomic causes included international disputes about foreign exchange and interest rates, and fears about inflation.
The internal reasons included innovations with index futures and portfolio insurance. I've seen accounts that maybe roughly half the trading on that day was a small number of institutions with portfolio insurance. Big guys were dumping their stock. Also, the futures market in Chicago was even lower than the stock market, and people tried to arbitrage that. The proper strategy was to buy futures in Chicago and sell in the New York cash market. It made it hard – the portfolio insurance people were also trying to sell their stock at the same time.[13]