MtGox has stopped allowing people to withdraw bitcoins off of the site. What this means is that in an attempt to get money off the site everybody sells their bitcoins to begin wire transfers. When you have a bunch of people selling and no one buying, the price drops.

Mtgox's prices often get discounted because they are isolated from the rest of the bitcoin market, as no one is able to transfer funds from the rest of the market onto MtGox, and vice versa.