One person mentioned earlier that the drops also happen on the weekend (especially on Sundays) due to no bank's being open. And I heard a rumor that a few big players like to move the market on the weekends by selling a ton of bitcoins and buying them back up. I can't confirm this. Simply things I've heard around the block.
This is actually an interesting theory. It makes sense as well... though I think there's a somewhat large element of risk there. When you do a mass sell to get the price down to re-buy, you are hoping that others will also sell theirs. If they don't, you lose.