The way my fWH described it, he was resentful, blaming me for all of the unhappiness he felt. The more he did that, the more he 'created' unhappiness that wasn't even there. He told me he wanted a divorce but was too chicken to tell me. He figured I'd discover his affair and end it, and he'd have what he wanted.
Well, he had it. I discovered it. I threw him out. That's when he realized what he had been doing, and that he wasn't unhappy at all, and that it became much much worse (the blame, the feeling of unhappiness in the marriage) during the A because that was the justification for it.
I can't say that this is what happens with all WSs, but I believe it is with many of them. Hope this helps.