It didn't really make sense but they had to create conflict somehow. I read an interview with Eric last week and he said that he was glad that the cheating wasn't random and that there was a history there. Can't say I agree with what he had to say that because it made it worse because emotional cheating was involved. Cheating is bad in general but imo it's worse if there are actual feelings involved than when it's just a random hook up.
BTW, I finished season 3 the other day, he looked so good, especially with a beard.
And I loved that he wore casual clothes, preferred that look to the suits he wore in the first two seasons.