I can only guess that at this point whenever the market sees it increasing against the dollar the majority of the market immediately takes profit by transitioning to USD. Perhaps also most accepters of bitcoin use it as a USD proxy, so the price of say socks on whateveracceptsbitcoin.com isnt .05 bitcoins, but is whatever number of bitcoins equals $2.00 USD, and the bitcoins are immediately converted to dollars after the transaction is complete.

I think those ideas may explain what keeps bitcoin from exploding above its current correlation to USD, what keeps it from radically dipping, I have no clue.