Oftentimes it's the result of the adult children never getting over how they were treated by their parents as a child. It's unfortunate we only get one set of "formative years". Things that were said to us tend to stay with us. Severe punishments are also never forgotten. If a parent was controlling or manipulative and never allowed their child to make any decisions of their own even as they approached their late teen years these are things that will cause them to want to escape.
When I left college I moved to California which was 2000 miles away from my family. It was wonderful to be "free". I didn't go back "home" for a visit for 8 years. Although I eventually had better relationships with both my parents I kept my private life private for the most part. I never had children so there was no need to keep them away from grand-children.
Many years ago I read a book titled "Toxic Parents" by Susan Forward. It helped me pull away from my parents without feeling obligated to put up with stuff I didn't agree with. In other words I was prepared mentally to cut them off for good if (they) didn't change how they interacted with me.
There were no ultimatums given or anything along those lines I just kept my distance and privacy. This led to them respecting me and treating me like an adult.