It's just a few impatient people getting out of bitcoins. They sold for a ridiculously low price for absolutely no reason at 15.9. Even though they could have sold at like ~16.7.
you might be right, it's back up.
but why would one do this? wouldn't it be easier to just sell in small amounts? plus considering the fact that almost all methods of withdrawing money from mtgox are delayed by days, this is just stupid.