I have read the post above and are all accurate in describing points that cause the door in self-closing.
A couple were left out I believe that could also contribute to this problem.
An interior door can be installed in a plumb wall and still have that problem in closing by itself.
The door can be installed with the hinge bound affect that keeps forcing the door to close by itself.
The door can be installed in a plumb wall, but the hinge side of the door jamb out of plumb slightly in the direction of the wall and still be square and functional and without seeing a marginal discrepancy under the door margin to the floor.
Not something I would write up, just a solution to the causes.
If a client wants it fixed, there will be other things that he wants to fix also.
I might just note that the door does not remain open and can be repaired by a qualified Carpenter.
Not really a defect, just poor quality of installation.
It would be called out in a New Construction build.
In a 20 year old house, I would not get to excited about the problem.