MTGOX is Bitcoin's central bank. when someone dumps of coins through MTGOX. MTGOX sells them at an inflated price because they know they have a monopoly on the entire exchange system. They are making money both on exchange fees and by controlling what the masses think Bitcoin is actually worth. Mark my words.
The problem with your paranoid supposition is the fact that MtGox is not actually buying or selling anything. They just act as a platform for other people to trade bitcoins, and take a slice of each trade as a fee.
Are you sure about that? I remember something else - at least back in 2011 they admitted to trade on their own exchange.