I've never *seen* it happen, but I've heard of it happening when they are underfed. Hermits are meat-eaters, so if they aren't getting enough, then they will either go after coral, or each other.
Feeds them seymore! I toss something meaty in every few days - i have a crew of hermits. Another easy way to feed, I drop in fish food pellets. Since they sink to the bottom, the hermits scamper after them.