This is probably just down to the door casing or lining not being plumb and so the weight of the door naturally makes it want to 'fall' open.
There's no great way to fix it to be honest, you could move the bottom hinge away from the rebate in an attempt to bring the door back in to plumb, the down side to this will be that the door won't close properly on the latch side. this could be remedied if you have linings by removing the rebate laths so that they fit the door again.
You could always fit a concealed door closer, I just put something behind mine to stop them from opening when not latched.
Hope this helps.