It's because the door is out of level,the top half of the door is sightly leaning over forcing the door to stay open or the hinges have a slight twist and not fitted 90 degree on the door or frame.Also the door frame could be out of level.Check the door frame is level,if that's okay then re-adjust the door but mean disturbing the door stop.You could add a internal door closer.
I have done this many time and seems to works,take off the door hinge and put it in the vice and squeeze it gently onto the pin so the hinge operate stiffer.
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