Looks like the guy butted him in the chest, which didn't look hard enough to make him go flying off his feet I wouldnt think, then he grabs his face when he got hit in the chest. I think the guy that got hit played it up bigger than it was but that dude is wrong for hitting him in the first place.
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"The towers are gone now, reduced to bloody rubble, along with all hopes for Peace in Our Time, in the United States or any other country. Make no mistake about it: We are At War now -- with somebody -- and we will stay At War with that mysterious Enemy for the rest of our lives." H.S.T. 09/12/01