Okay, since I am no longer a Republican, I can admit I was wrong for the most part.
Upon further review, it seems that the AFP gang was itself about to take the tent down and some of the union members let their enthusiasm get away from them and decided to help out.
At the beginning of the Breitbart clip, a man shouts, “there it goes!,” indicating that the tent was coming down at that point, yet we hear no shouting, cursing, or other signs of a real confrontation. A little later, the AFP guy (tan jacket and white badge) seems completely unconcerned with the union members pulling and tugging on the tent, nor are they paying much attention to him. Note also that the entire enormous tent structure collapses almost simultaneously, which is unlikely to be the result of a mob attack randomly slashing and pulling at the ropes.