As far as I've read, it was a bunch of quantitatively managed (run by software) hedge funds all trying to cut their losses by selling at the same time. Once people realized what was going on, humans took over and swooped in on the bargains, pushing prices back up.

That doesn't cover everything though. There was some weird stuff going on with the central bank of Japan. The Yen did very well against the dollar at the end of last week. There are rumors that Japan is going to dump US treasury bonds. (Never mind that last, grabbed the wrong link, that's old news, and unrelated.)

Tomorrow is going to be interesting.