its the whole impulse control thing... .they either mess around, hurt themselves, drink, gamble, shop etc.
They can't control their base impulses anymore than they can control their emotions
This, coupled with the fact that they cannot stay alone - remember, they have a deficient self, thus they need to be in some relationship to feel complete - plus their fears of abandonment, which drives them to find potential replacements in case something goes wrong with their current partner.
That's why, imho, we observe infidelity in lots of relationships involving BPDs... .they may react impulsively to some perceived wrong, and among the impulses there's the cheating thing.