Something I noticed here, Beth talking to someone on the phone after Cathy's cluck incident, the other people at the party are looking at the phone as if seeing Rooster and commenting on it, in this render we see Beth informing someone that the job was done?
JB mentioned that he heard someone fucking in Rusty's room when he passed by, one hypothesis is that Rusty hooked up with Beth before, Beth thought it was more serious than it really was, and when she caught Rusty with someone else she got angry, and decided to take revenge. (Calling Rusty a manwhore isn't much of an insult, unless the person saying it was a woman with a grudge.)
After what happened with Chad he was basically kicked out of the frat, perhaps she expected the same to happen with Rusty, but as nothing happened, she decided to act against the DIK's again in this update.