Is it possible the SO's are just unstable people?
If you think about it they couldn't make it work with the parent of their children, the person whom they choose to have children with. Most relationships have shelf lives. However when you have children with someone you really try your best to stick it out no matter what because there are other lives involved that could be torn apart by it. There is also a special bonding that occurs with the person you have children with. So they ether failed to do that for one reason or another to make the relationship work that was important on so many levels or they choose terribly with whom to have children with which leads me to believe they may have some serious issues themselves. I found my relationship with my SO was full of conflict involving either his kids, his ex or his mother. He would believe anything his kids said automatically without question even though they had a history of exagerating, manipulating and lying. He also would push and pull. One minute I was the love of his life and the second mom to his kids and the next I was flawed on some way, not worth listening to and treated with anger and discard. Which makes me wonder if the issue is the reason why they were a divorced parent to begin with was because they are not emotionally stable people and have no business being in a relationship unless it is unhealthy and the other person is willing to put up with their behavior because of their own issues.