They are not overtightened or obstructed from sitting on the door how they are supposed, that was my first assumption. Not hitting them with anything, dropping bumping or anything like that, car isnt even running right now, waiting on turbo.
The first set i purchased used were cracked, minor hairline, so i epoxied prior to install to make sure they were good and strong. well they cracked and became loose farther down, where i did not glue. purchased another set of used but in better condition no cracks but heavily glued anyways to be safe than sorry. havent driven the car since install, open door and shut there it went loosened up pulled off and cracked!!!!! I dont think it is the glue, but guess it could be possible that the 20 year old plastic might be gettin weak from that along with the weight of them heavy bastards? Its actually plastic weld two part epoxy.
I am tired of buying used mirrors and cant afford to buy brand new at this time