I think because Paula talked about the prisoners the terrorists wanted to free in Bengahzi. They let Petraeus know they knew about his affair and that he would need to go. They did make Petraeus wait until after the election though. That’s my take.
I remember that Paula’s father came out and said this whole thing was not about Petraeus but about something else that we would all know someday. I think he hid her hard drives and data, IIRC.