Why do they stalk? Fear. Insecurity. Knowledge of how easily an A can happen. Maybe it is a leftover habit as well. It would seem that the spouse would be a constant thought on the mind of the AP, comparing, analyzing the M, etc. Also, What else does an AP do with their time while the WS is with the family?
It is such a screwed up way to live. Truly, it is hard to believe that anyone would allow themselves to be the "other" person, but hey, turns out there are a lot of people who do choose this.