If it wasn't tightened down when installed, it would slowly loosen until it came out. My experience with this was a 351 Windsor I installed in a Ford truck a long time ago. Got it on the road and left for PEI the next day. On the way back from PEI all of a sudden the engine was making a wop wop wop wop noise. I pulled over, popped the hood and here was a plug wire with the plug in it laying off to the side. I screwed the plug back in, checked the rest and drove home. Never had another problem.