Nobody reasonable who just wants to "sell their stash" dumps it on the market like that, as they could sell it slowly and get more money for it.

It someone dumps a big stash like that, its either 1) they want to deliberately crash the market to buy back at a lower price, or 2) needs fiat really soon desperately, but then they are not gonna do it in MtGox anyway as delays in fiat withdrawals are huge, so it leaves us reason 1). Or they are clueless how bid/ask markets work or careless about their money, but anyway anyone who is highly reasonable and money-wise will not sell a large stash like that (unless trying to deliberately crash to buy back) as they can easily get significantly more by spreading their sales evenly over a day or two.