That's why the PPS at these volumes doesn't mean much. The action could just as easily have went the other way where half a million shares hit the bid and the PPS recovered by someone taking a few thousand on the ask.
As far as your supposition that someone payed more than the ask, I think you need to look at the trades and how they work. The shares at the lower PPS were exhausted and the buyer hit the higher ask. Nobody payed more than the ask and nobody went short. The bid simply dried up and a few shares went off lower.