The hinges are welded to the 'A' post of the vehicle. Or at least they are supposed to be! The 'A' post is a double skinned affair, but in some instances the factory weld does not appear to have penetrated through to the second layer. What happens is that the single skin holding the door becomes weak with metal fatigue and eventually cracks and allows the door the fall away. If left indefinately the door would literally drop off its top mounting. Go out and check yours by opening the door to 90º and trying to lift the door. if any movement can be detected where the hinge is welded, or any cracking or weakness is evident, get it welded through to both skins. It is a very fiddly job and should be undertaken by a competant welder. this fault appears to be more common in estate versions.