at first i thought a disagreement sparked this, like they probably made some wise crack to her and then got mad when the boyfriend defended her or something. But it turns out they were actually on the prowl for a woman to rape, the driver of the bus was in on it. They didn't even care that the guy was there, they just knocked him out and beat the shit out of him, basically incapacitating him so they could then attack the girl...christ i've seen a lot of fucked up art house movies but nothing comes close to this, not even Irreversible.