The police would state afterward that he was being beaten with his own baton. But the video below shows everything up close: he was shoving his way belligerently through a crowd using his baton, which he negligently drops. He then freaks out and pulls out his gun, pointing it at the unarmed student protestors.
I have been in a number of protests of similar climate, where the protesters are trying to hold a line while the police try to shove them back. If there are enough protesters, and they are locked arm in arm, then they are difficult to shove back. The police then either back off, or get more violent, meaning charging the line with batons out, whacking who they please, or pepper spraying anything in their way. The police want to make the protesters all go home, so they back them up, then back them up some more. That is what you are seeing below. Here are some still shots of the video footage.