I lived through this as an investor. That evening the "experts" were all calling for the DOW to drop from about 1750 (it was about 2300 before it tanked)to 1000 or less. Instead the futures turned sometime during the morning of the day after and the market began it's climb back. No one really knows the cause of this one day record decline. here is an interesting link for you to examine.
http://www.lope.ca/markets/1987crash/
Great link and thanks!