I´m not sure about Mt.Gox coins. But if you look at the orderbook, those 100BTCasks get refilled all the time.
Someone with huge pockets full of BTC is actin(sellin).
There is also the possibilty that someone is buyin his own asks,to unleash some bullish sentiment, cause it looks quite unnatural.
No trading at all , just those 100BTC buys in one bite.
As far as we know there are quite a lot of stolen coins out there, just think about those 100k sheepmarketplace coins.
It takes time to convert these into USD. With that amount of coins you can play the market in every direction.