Some people don't realize what parenting means / entails and they grow resentful. Some couples can't handle the responsibility and it ruins the relationship / makes them go through the motions. A part of those will blame the child instead of themselves for creating a life.
There could also be personality differences / two stubborn people bashing heads.
Usually it does mean it is a pathological family. Kid leaving at 18 is only okay if it is their choice that isn't caused by bad relations with the family, and for whom the door is always open to visit, or even return.