One of the issues is that some adult children revert back into the role of child versus their new role as "adult" child. This may include not contributing to the household and expecting their parents to provide for them as if their were a "child" (not doing their own laundry; expecting mom/dad to cook meals; not helping pay bills, etc). This leads to some adult children taking advantage of their parents.
If the adult child and parents do not discuss expectations ahead of time, there will be problems. Sometimes they assume everything will work out, because they are family, and of course...that is an automatic to successful living! Not!
Well, as a person begins to get older they start to demand a certain amount of "independence" from others. And that independence sometimes can be a barrier between parents and their children when they begin to get into adulthood, or even when they are older. When a parent offers help, and the child takes it, then it can lead to some feelings of resentment towards the parents.
Sometimes the child feels like the parents let the child go down a slope of things in life that has lead them to this point, thus making the parent responsible. This happens usually when a child has problems with responsibility.
Also, the child may want to be "living their own life" and not having to put up with their parents.
This may remind the child of when they were younger and had an adult constantly peering over their shoulders to see what they were doing. They may feel as though that they don't want to share the same "space" with someone that they feel still has authority over them. Many cultures see a parent as always deserving respect from their child.
Also, the parent may be why they feel alienated. The parent may think that they have the right to demand,"Where were you?" and etc.
The parent may also think,"Well if you're going to live under my roof, then you're going to obey my rules because, after all, I am your parent!"...