Shirley Glass noticed a fundamental shift in dynamics that tended to precipitate affairs. Usually couples have a shared intimate space in which they can exchange confidences, daily updates, and occasional relationship complaints (which actually help to maintain a strong relationship). It is as if the couple builds a wall around this shared space that keeps others out of their intimate relationship space. 

However, affair-prone partners tend to reach a point in which they stop confiding in each other, stop sharing daily stories and stop raising their relationship complaints. In effect they close off this shared space and replace it with a wall.

They may then meet a friendly person and, over time, they may find that they are able to confide, share daily stories as well as their relationship complains that they are withholding from their partner.

In effect they are opening up a new shared space between them and this new person, which becomes more and more intimate. Eventually the initial walls may become reversed and they end up building a wall around them and this new person which keeps their partner out.