Sometimes I think it might be the snail's fault.
No really... I've noticed lately that some of my first trochus can hardly stay upright anymore. Sure, they fall over and can't get up from time to time, but it seems like there's a point in time when they just - get old. And that's when they fall prey to the rest of the tank. I haven't lost any snails to crabs, but I've seen my itty bitty nassarius snails get in there and have a feast on a snail I haven't gotten back up in time. They literally crawl into their shells. I think if the big snail was stronger, they'd just "flick" them off, but sometimes it seems like they just give up.
Of course, it could be a food issue too. Maybe there wasn't enough for the nassarius to eat, so it went after another snail that was easy pickings? But it doesn't sound like that's the issue for your tank. Sounds like you keep your crabs well fed.