What happened? Did the market sneeze, cough, or was something misread and today perceived in a different light?
In my opinion this is what happened:
The Plunge Protection Team, as they have done on previous equity market drops, or the Federal Reserve operating for the Working Group on Financial Markets, sent a purchase order for S&P futures to the trading floor. The hedge funds, seeing the incoming bid, front-ran the bid by stepping in and buying S&P futures. This pushed the market back up, ended the correction, and prevented financial panic.
The Plunge Protection Team was created in 1987, approaching the end of the Reagan administration, in order to prevent a market correction from costing George H. W. Bush the presidential election as Reagan's successor. The Republican Establishment was desperate to reestablish its control over the party.