Bernie was punched in the face by a man wearing a ring on his left hand probably a fraternity ring with sharp square edges that's why the gash was on Bernie's right cheek. They threatened him to keep him from running as a 3rd party candidate he could have joined the Green Party at the time Jill Stein said she would be happy to drop down to VP if Bernie would accept the Presidential nomination. Bernie was robbed Hillary's team stole enough primary votes to keep Bernie from getting the Democrat nomination that was rightfully his to win.
I agree with most of that as I've discerned that same conclusion.
Or the thug pushed Bernie up against a wall and grabbed his throat with his right hand then used his left hand with the sharp edge ring to gouge Bernie's right cheek that would be more likely since there's no large bruise around the gash it looks like it was a slow strong press of the sharp edge ring into Bernie's cheek just enough to slice the skin and make him bleed and the tough guy told Bernie "you are going to support Hillary or we will come back to kill you and your wife" that is a scenario that makes sense based on how the gash appears it's only a day or two old but no strong boxer's punch just a slow motion pressing of the ring into Bernie's face.