One explanation that I've heard is that people run bots which query the MtGox API. The bots are programmed to sell if the lag gets too long, because without information on the current price, that's considered the safe thing to do. So when MtGox gets a huge lag because of high trade volume, or in this case because of a DDOS attack, that triggers the bots to sell. Of course, that doesn't explain all of it, but when people see the price dropping, then they panic and sell too. Supposedly that's what happened during one of the other sell offs back in March. Not the one that went from $48 to $33, that was caused by the block chain hard fork. There was one before that which was also triggered by MtGox lag.