You likely saw the gox API glitch caused by a 10k sell. The API repeated the information like a loop for quite sometime making it look like far more was being sold than actually was.
Ouch, that's still quite a lot... Looks like 1/3 of SecondMarket's Bitcoin Investment Trust or another huge fund. That means more selling may come.
There are actually plenty of people with that many coins so no way to be sure. I don't think it would be SecondMarket, at least not their customers/clients as they have a no sell rule until next summer I believe.