If it's any consolation, Twig and Rocket(mostly Rocket) do it too.
Rocket usually goes after Isis and Luna and pins them down and bites the back of their necks and stands over them. Isis usually screams, but then when we come and break it up, she is fine and there are no marks. I think since she is skittish, it scares her to have this happen to her. Luna just sits there and waits until rocket is good and satisfied then he lets her go and everyone is happy. Rocket is competing for the alpha cat spot with Twig,but since twig is 15 pounds, Rocket can't quite make it into the alpha role cause he can't fight off Twig when Twig decides he needs to be put in his place. Fortunately Twig never hurts any of the cats so I think Rocket is more humilated then anything so he will take out his dominance over the girls. Long story longer, Don't worry, as long as no blood is being drawn and noone is being hurt, then just break it up like I do.
It'll sort itself out sooner or later...