Many reasons, which i believe always has selfishness at its core. Especially when kids are involved.
Know of a case where the girls mom was all mad that her daughter was “taken away” from her. She did everything to destroy the marriage. Her daughter wasnt strong or willing enough to deal with it or even see a marriage counselor. They eventually got divorced. The guy showed me two years of his cell phone bills of how his wife was on the phone to her mother EVERY night for over an hour with no phone call beginning before 10 30 PM….