Research into abusive behavior by ex spouse?

My partners ex wife left him for someone else. He had no idea she was leaving but did not oppose her decision. She supposedly is happy with her new life. However she is abusive/ is demanding excessive property settlements/ is obstructive regarding any family access to their child. To put it bluntly, she and her new husband are cruel. If she wants to move on, why make an ex husbands life hell? Is there any research into adults behavior post separation/ divorce? We would like to understand more about what motivates such extreme behavior. She is unable to remember a friendship let alone being married for 12 years.