My ex told me because "he couldn't be treacherous any more" and said he was seeing OW. I've since found out that it was because OW had told her husband that she was seeing someone else! However, I've certainly assumed that that was BD, even though he then almost seemed to be trying to make it up with me (at the same time keeping in contact with her!). I didn't know it was MLC then so told him he had to go, which he did three weeks later when they moved into a flat that he had rented.