4. She actually does believe she can do more for Alaska and America by leaving office in mid-term. (No one believes this; it's simply mentioned here for the sake of logical completeness, and then discarded.)
3. Pregnancy. (Several variations on this, most circling back around local rumors from a few years ago.)
2. Governing is complicated and boring; what she really likes is the money, the fame, and the platform for score-settling.
1. Indictment(s) in the works.