Somebody sold their stash, I guess, much like what happened on June 2th. This thread belongs to the speculation subforum btw.
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.
Unless, you know, they didn't know this. Just because someone had hundreds or even thousands of BTC it doesn't mean they were aware how markets work.