Someone else mentioned panic and that I suspect is right and part of the source for bitcoin that differs from traditional exchanges is that it is a much more risky currency at this point. It is very susceptible to panics, and rightly so. I don't think anyone could seriously invest in bitcoins; it would be the dumbest thing you could do with your money. You'd be better off eating your money and get the caloric value. I'm just being honest about it; maybe it will change one day.

People buy less, so those wanting to sell have to lower their prices, which makes others panic and sell at a low price, and there you go.