from...

from MarketWatch.

http://www.marketwatch.com/story/us-stocks-end-lower-after-nightmarish-day-2010-05-06

"The panic in the middle of the day was market makers that just disappeared, and every machine on Wall Street was trying to sell into a market that didn't exist. That was a bizarre electronic quant panic of people selling into a black hole. I don't know if somebody pressed the wrong button, but the intraday was a panic machine-led move," said Peter Bookvar, U.S. market strategist at Miller Tabak

so it would seem they had a computer glitch.