You shouldn't try to hit people.
Lars is at the baseball game with his son Kirk and Dave is being really loud jeering behind them and being generally obnoxious.
Lars wants to get Dave to quiet down.
Lars turns around and tells Dave that he's with a child and Dave really needs to tone it down.
Lars stands up, turns around and takes a few punches at Dave, landing some on Dave's face and head.
Dave is apologetic and tells Lars he got carried away by the fandom, and he'll calm down.
Dave is freaked out because he didn't expect violence and is terrified as he can't really defend himself in his position.