With the very high price of bitcoin last days, I suspect they have a problem when someone wants to cash in a larger BTC accounts. They just don't have that money. They force the price to drop... Happens ones, happens twice, it will happen again.
Mt. Gox is an exchange, if you want to sell your BTC for USD there must be a willing buyer available. Mt. Gox only role is to match buyers and sellers.
Normally, every buy/sell order only happens in their system. Probably, the real money is invested somewhere, like every normal bank does (they want to make money, don't they?). If however, certain people with large BTC accounts are announcing that they will cash, Gox has to reclaim that money. Delaying trades (faking lag) and thus dropping the price redistributes the bitcoin to people who are not intending to cash on short term. The guys wanting to buy-out just get paid less...