MK had to be forthcoming to AF, because of the kids. She had to tell him where she was moving, Etc On account of custody issues.This is JMO, but if I had to fault MK for anything, it would be that she was probably more than forthcoming to AF about her future plans, especially if her new location included the fact that she was going to move in with another man. If she was pregnant, AF might have figured out that he wasn't or couldn't be the father. All just more motive.
MK was in a DV situation. From what I've seen if they don't tell the abuser what they want to know they beat it out of them. Easier to tell him the truth than take a beating.
AF probably threatened to harm her family. Knowing the abuse she was taking she wanted to keep her family out of it, Its just so sad.