The first action in the video, he puts his hand on the players back - happens a bunch every game - he's telling the player 'I am right behind you'. A second later, the player turns and literally tries to run through the ref, instead of going around him. You want him to get out of the way? The kid was what, maybe 1, 1.5 yards away at most, and ran right at his middle. Would YOU be able to get out of the way in time? The ref has right to be on the field too, and it is generally expected that players try their best to avoid laying out officials - who aren't wearing pads - in the middle of the field. It's hard enough being a U where players are coached to chip you intentionally or use you as a screen or shield, and some just don't like officials and want to make life hard. I don't know why that kid ran right at him but that action is on the player, not the U. He was where he was supposed to be, he let him know he was there, and almost got trucked for no reason.
I don't excuse or condone what he did, in any way - but I can certainly understand why he was irritated at the kid. He'd have been better off lowering his shoulder for the hit from the kid (which happens a fair amount also!)
As far as injuring the kid - it was literally no worse than any other FMM foul that happens every week, every game. I don't see any pearls being clutched when helmets get popped off during the course of a game, by being yanked like this, and we don't call it assault either.
/off soapbox