People cant get dollars out of Mt.Gox at the moment - which means more people want to buy BTC on there (so they can withdraw) and less people want to sell (because the can not withdraw the dollars they receive).
Basically, a dollar on Mt.Gox is not actually worth as much as a dollar on other exchanges - so it takes more Gox dollars to buy a BTC.
Exactly. Because people can't get their USD, EUR, AUD, etc., out of MtGox, they are dumping those and buying BTC. While it might seem like selling your BTC on MtGox is easy money, you have to consider that it will be stuck there for months.