Andrew is right on the money.  They were probably either (a) abusive, (b) under the influence of unnamed substances,  or (c) repeatedly refusing to follow instructions.  Or a combination.  You want those people sorted out before the plane is in the air.  So good on United!--  I have no criticism of this. 

This is no joke, and there's a safety issue involved.   I was recently on a Delta regional jet flight where they had a bit of a challenge balancing the plane before takeoff.  The flight was maybe 60% full.  The cabin staff horsed around in a game of musical chairs for a few minutes without much success.  Then the captain came back, and said, 'Just move everyone in the back into Premium Economy.'  Seven people moved up front, and then we were away to the races.

Bottom line:  (a) You are not the boss of the plane.  Do as you're told.  (b) It never pays to annoy the cabin staff, although I would prefer to have said that in a more colorful way.