Mystery of the overhead light

Those of you who have followed this blog for a while will remember that my wife and I moved into this house back in 1979 we had a terrible time keeping a mirror in the downstairs bathroom wall over the sink. That mirror would wind up on the bathroom floor (once it was found in the hallway outside the bathroom). It was never broken, though you would have thought going over the porcelian sink would have damaged it). We finally gave up because no matter how securely you put that mirror up; down it came. So I placed a full length mirror on the door for my wife. Several years later I came into an unexpected small inheritance and my wife and I decided to replace the old bathroom. It had been added to the house as this house was built in 1857 when such plumbing was outside. It wasn't hard to remove and we replaced it with a nice modern structure. My wife insisted on a fairly fancy medicine cabinet with mirror and, frankly, I was apprehensive about putting it up over the new sink. But it stayed in place and has now for several years. But that is not now the problem. This new bathroom has lights on either side of the medicine cabinet and an overhead light. Actually, if I had known then what I know now, the overhead light would have been excluded. The side lights would have been quite adaquate. An electrician who wired the bathroom first installed the overhead light. Everything was great for the first few weeks. Then I noticed the overhead light was askew. I took off the globe and inspected it. I found several loose screws and remarking to my wife I was surprised the electrician was so sloppy in his work on the light. I tightened the screws and everything was fine for a week or so and then that light was again askew. This went on for several months. I got tired of continually reattaching the overhead light and I started really ramming those screws home. Didn't help, within weeks the light was askew. I just let the blamed thing go and it slowly went further and further askew until it was hanging by its attachment wires. So today, I hauled out the short ladder and discovered the long screws that secure the base of the light to the crossbar that is securely screwed into the ceiling joist were totally unscrewed and lying in the insulation in the lights base. How could screws that long completely unscrew and be lying in the insulation? I put the light back up. I screwed those bolts so tight I bent the bar. But I really expect to see it dangling off the ceiling in a week or two. Any contractors out there tell me how two long bolts can come totally unscrewed all by themselves? My wife says that whatever was playing games with the mirror is now playing games with the overhead light. Maybe she's right.