@Allyn Storm left because she's on Scott's Extinction team, that was her decision during Schism. You really should read it and ReGenesis. She left because the Avengers were planning an assault on her home and to extract a person she has fought to protect. As I have mentioned before, this event deals with issues and plots that go back at least to when Bendis took over on Avengers and for the X-Men maybe back to Morrison's run since Schism and ReGenesis have touched on many creative runs. I would suggest reading those first because you won't really understand most of the stuff going on here.
Marvel hasn't done a good job of making this event reader friendly for those who aren't familiar with the two teams. They're doing a rudimentary job of explaining things in like one dialogue box when really a fan needs to go back and catch up themselves if they haven't been reading. Just my advice.