Remember that 1000 point drop in the dow for a short time one day that was blamed on someone fat-fingering an amount. My brother and sister both work at different financial firms. None of the pros bought that explanation. It was ludicrous on the face of it. But apparently the media was instructed to let it go.
Which begs the questions, what happened and what caused it to mostly recover?
Well.... most of the volume in the stock market is controlled by high-speed software, the purpose of which is to make money by taking advantage of short-term price fluctuations; and also to minimize losses if prices start down. Transactions are computer-to-computer.
Although I'm sure the programmers try to put in safeguards, I also know that they can't think of everything; and eventually things are going to line up just right so that the software fails in some highly-noticeable way.
I've always figured that's what happened here.... and I would not be at all surprised to learn that somebody had recently dropped a new software version onto their system, too.