The current big news is that certain exchanges (ie. Mtgox) are beginning to suspend withdrawals of currency for various reasons. There are talks of Mtgox declaring bankruptcy (but don't take this as fact), so that scares people. Also, certain government(s) are beginning to ban cryptocurrencies to some effect. Like currently popular currency exchanges, negative viral news affects the prices dramatically.
Mtgox and Bitstamp suspended withdrawals due to a vulnerability. People have to sell in order to withdraw funds.