I recently had a similar situation where one crab dug up the molting crab, but thankfully there was no attack. I feed my crabs proteins every day, so I believe my situation was an act of aggression, possibly due to the substrate not being deep enough at that time. My crabs especially like hard boiled eggs and shrimp. I also give them tuna, sardines, chicken...... of course everything with no additives. I also tried ground crickets, but I don't think they cared for it. So protein every day, not as a 'treat'.
Please post here if you are having a crab care emergency! Use a real subject and not just "HELP!"