It looks like there's a video of the actual shooting.  The "victim" and about four others surrounded the shooter.  It looks like the shooter had his arm bent back behind him, not even looking the communist he shot.  I think it was a weapons retention issue, like his gun was being taken from him and he pulled the trigger either accidentally, or in an attempt to not have it taken.  Clearly the shooter seems like a shithead for bringing a weapon to a place like that (most open-carriers would avoid scenes like that), but the communist ran up to the shooter and several of them wrestled.  The shooter was initially blind-sided.  Watch the video.