People need to cash out slowly. Imagine, if just 2% of all people cash out quickly (or everyone just sells 2%). That would be 240,000 coins hitting the exchanges. Look at the order books and make your own guess...
Remember that if someone is going to dump 1000 coins on Gox now, we would approximately go down by 40 USD. The guy doing that would have pocketed around USD 880.000, but the Bitcoin marketcap would decrease by USD 480 Mio.
EDIT: (not implying that one person can move the market as such, I just want to point out how fragile the whole wealth in Bitcoin is)
No market of any size can support immediate liquidation of 2% of the entire market value.
If people tried to quickly cash out 2% of the world stock of gold, the price of gold would crash.
If people tried to quickly cash out 2% of all housing, the housing market would crash.
If people tried to quickly cash out 2% of all gov bonds, the bond market would crash.
If bitcoin is a store of value, 2% won't be quickly cashed out. The market does not have to support that.