Because she most likely never broke all contact with her OM. It is an addiction, and it's extremely hard to break.
And it is a symptom or a coping mechanism to the crisis. As is alcohol, drugs,over-spending, promiscuity, ....
The Affair and her behavior is "what" is happening.
The "why" is crisis.
Until she deals with the crisis your M will be in limbo.
That's one theory; I happen to disagree. There are plenty of so-called "mid-life crises" whose behaviors are simply that of someone addicted to an affair. What came first -- the chicken or the egg -- has been debated (and researched), and I have yet to read any conclusive evidence one way or another.
At a MINIMUM, someone caught up in a life crisis, who is also having an affair, will be strongly affected by each contact with their affair partner. It's physiological fact that such encounters stimulate the brain with endorphines called PEAs, and it makes it very difficult for any move back toward the marriage as long as they keep getting their "fix" in this way, regardless of whether or not there is an over-arching life crisis going on.
