If your engine mounts are getting mushy, propeller thrust will push the engine forward - along with the shaft and the seal ring. If the seal faces open, ... well, you get the picture.
Running a vent line to let air out of the seal is a good thing - they certainly don't like to run dry. But just pulling the seal faces apart momentarily to 'burb' the seal will do the same thing. Another trick (which I've used on some big face seals) is to provide them with a source of filtered clean water. Tap into the sea water pump discharge, through a filter, then a small line to the seal. Keeps the grit from getting to the seal. According to the outboard folks, the worst water for seals is the glacial rivers in Alaska. The white stuff in the water is ground up rock, which eats seals! The outboard folks sell special water pumps for those places.