When they showed the replay, the PU player was on his back (or falling down) facing the other way to the action, and his stick hit the ice, which the UMD player tripped over. It didn't look like he intentionally tripped him up, just looked to be bad timing on the PU player's part.
That said, in a normal game situation, that should be a tripping call regardless. But when the refs had let numerous infractions go earlier in the OT, if I were in Princeton's skates I would wonder why they chose to blow the whistle at that point.
It doesn't bother me either way, I'm just saying that it would leave a bad taste in my mouth if I were a Princeton fan/player/coach.