I guess its about how the other woman made her feel (or so she thought). In all our relationships we forget how lucky we are that we have this person, or these people in our lives. Over time we become comfortable with our lives and with the people in them. We also take the people in our lives for granted, many times pushing the boundaries for what a relationship is worth. In your story, the couple must have fallen into that trap, when the other woman got involved she re-directed the married womans attention; making her feel wanted, different, the same way she felt when she first started dating the guy.
She left him because he wasn’t either paying her any attention, or was no longer the guy she married. Remember if your in a relationship and you got that someone because you impressed them, then you have to keep doing stuff like that so she / he stays interested and doesn’t feel like their being taken for granted.
Thats my opionin anyway