1. She's doing it just to make him suffer, as shitty rivals / antagonists do.
2. She's doing it so she can keep him away from her 'friends' out of general disdain for him due to recent events.
3. She's trying to get all other girls away from him due to some one-sided lengthy crush she's been hiding, in a 'if I can't have him then no-one can' type deal.
Oh, and if its any of those three, she's probably also the one responsible for the other candidates just up and leaving, to further her ploys.