My guess, he’s a serial cheater. She put up with it for decades to keep the marriage. Then he stopped being discreet and flaunted his sluts in front of and expected her to be nice to them but that was a burden she could not bear.
He's a REPUBLICAN governor who just won re-election. You think any cheating by him would not be uncovered and used by Dem operatives?
Democrats can bang young interns in the Oval Office and engage in forcible rape, and the media will refuse to cover it. A Republican can't make an off-color joke and not have it be 24/7 coverage for the following year (ask Clarence Thomas).