Certainly rule out medical first. Does he show pain when you manipulate his ears other times? Or only with the snood? Does he dodge away ffrom the snood when you put it on? Try putting it on in a different situation unrelated to dinner and what does he do?
I know he's shown food aggression towards Shelby before, but this is odd because it's AFTER they eat. And in the past, has he ever directed this behavivor towwards you without provocation from Shelby trying to get to his food or anything??
It's odd, for sure. Check his ears, around his head, check for wounds, foxtails (you know how bad they are down there! he could have one stuck in his ear and causing pain -- but then you'd expect to also see scratching at the ears or other indications of discomfort). I'd ask the vet to check his ears out anyway. Could be PSOM but that is not always easily diagnosed. Only if it's really bad and the pressure and mucus behind the eardrum has built up a ton, causing the ear drum to bulge out. You said you haven't seen any other symptoms of SM, right?
Certainly see what he does wiith the snood in differernt circumstances. It's not just excited biting at the snood like Rory does when I put his on or when I put his harness on. It's aggressive biting that is causing pain and bruising on you. Eventually it could break skin. So out of character...
So yea -- I'd suggest ruling out medical problems, first. Then see if you can isolate what is causing this behavior. Feed him totally separate from Shelby. In a different room where he can't see you, etc. It's just odd... I dunno, Cathy.
Dogs are not our whole life, but they make our lives whole.