"There must be a point when the small shake of the camera is over come by the long exposure. No?"
You would think so, but that's not the case. Highlights in the scene can leave a ghost in any "knocked" double exposure caused by a poorly covered/uncovered lens. I used to have the nasty habit of closing the lens by lowering my cupped hand in front of it, and would sometimes hit the top of the lens before the front was entirely covered. That resulted in double exposed highlights in the bottom of the frame even after exposures of a minute or more. It doesn't make a whole lot of sense, but I've got plenty of ruined negatives to prove it!