I might have responded with this to one of your past posts.. but my car threw that belt once right after me changing the belt. Ever since it was towed to the shop and the belt replaced the problem hasn't happened though the alternator slips during the winter. It seems like the shop set the belt slightly loose but I refuse to touch it since everything has been working. I think this HAS to be due to the engine moving, might be time for engine mounts. Either the pulley system is shot (I checked it to the best of my ability and found nothing abnormal) or the engine shifts which twists and shreds the belt. I'm guessing with a little slack this won't shred the belt but it might slip and get a bit glazed. With a tight belt the force just destroys it. I remember when my belt shredded half of its width was completely gone and had turned to pieces of rubber all over my engine bay, while the other half was partially over the pulley. This seems to imply some sort of twisting of the belt ( the belt was also rather twisted).
Just a hunch but engine mounts might be a good idea.. though with the maintenance you do you've probably already done those. If someone with more knowledge has an idea please share.. I hate when something as simple as a belt disables and overheats our E34s.