Hopefully your crab just ditched the limbs due to getting them stuck/injured or being frightened somehow, although I would also doubt the clowns would be able to do it unless they're quite large by comparison. Unfortunately, I would think it could also be a bad molt in progress. The only crustacean I've seen that ripped off a bunch of its own limbs that fast was a mitrhax-family crab having a bad molt that it didn't survive. In that case, from the discarded limbs it was apparent that the old skin was quite thick (too thick for a normal molt) and the new skin was already hardening under it.