This one looks relatively straightforward. Obama wanted to get through the election, and he did. Now he is housecleaning, to avoid having someone FORCED to resign under his tenure, which would look bad politically. What we can infer, I think reliably, from the timing, is that not only does Obama have an enemies list, but he is also actively gathering dirty little secrets he can use to blackmail people. These are pretty basic Communist tactics.
What I would submit, though, is that IT DOES NOT MATTER that Hillary and Petraeus resigned. They still need to be forced to appear before Congress to answer questions. Their jobs are no longer on the line, but four men are still dead, and we still need to answer basic questions about our foreign policy, and we need to know what Obama knew and when.
Hopefully, Congress, Obama does not have dirt on all of you, and hopefully you still have a pair. Do your fucking job.