It would surprise me if Mt. Gox would maliciously manipulate accounts, as this would not make sense for them. Currently they own loads of money via transaction fees - why should they risk their monopoly status for some short term gains?
Perhaps because they think that there are only short-term gains to be had out of bitcoin? I'll take a hundred thousand real dollars now over a million bitcoins in 2013, thankyou very much.
MtGox has been fundamental to the whole pump-and-dump side of bitcoin so it wouldn't surprise me if they're manipulating the market.
