There are a number of boards that do this when flexed. Some sockets also let th legs "creep" a little on repeated heating and cooling (like powering on, playing for a while and then power down) and peiodically you need to reseat the chips that have creeped. My Battlezone is like that. Once a year I press all the chips down on all the boards. Still kicking after 27 or 28 years.
ken