This had/has been happening to me too in a hotel I had been staying in with a very very bad wifi connection (to the point where it would connect and disconnect over and over. Not a very tech savvy hotel).
I have since left the hotel, and haven't noticed that it happened.
So my unscientific explanation for the problem is that if you leave the iPad near a bad wifi connection it gets confused and freezes itself in it's sleep.
But I really have no idea. I would love to actually know.