From that I can recall portfolio insurance was the new strategy being used by banks. Whenever the market dropped, people sold the S&P500 short as a hedge. This was a good strategy until everyone did it. The programs they’d written kept selling the S&P short when the market dropped and when the other banks software, saw the market continuing to drop (due to those short sales), they sold the S&P short too
It kept going on and on to the bottom.