Seeking another job (presidential run, fox news host):
Quitting a governorship mid-term leads to the label "quitter" and the concern "if she would quit the governorship because things got tough, what prevents her from quitting as president".
Moving to a non-public-service job (Fox news) is the kiss of death for any future run at office.

(It seems unlikely)

Scandal:
This would have to be a scandal of the form/size we haven't seen before.  The standard response for governors and statesmen is to hunker down, deny everything, rinse and repeat.

(I just don't see it)

Family emergency:
We are not talking about pregnancy, here, she has shown that she is willing to run for office while being pregnant.
We are talking about something on the level of cancer, Tony Snow levels of cancer to quit so suddenly.

Certainly anything is possible, especially with someone who has such high levels of maverickyness as she, but I don't see "leaving mid term" as being an irrational decision, rather she has all the pieces to the puzzle and her actions somehow logically fit.

Lastly, has anyone considered how erratic Palin's last two months have been in the media?  Maybe this is more evidence of a strain on her/family that might help fill in some of the empty areas of the puzzle.