HCA: Hermit Crab Association

I just found a crab leg sitting by the food dish! I read in a post losing a leg can be a sign of stress. About 10 days ago we changed the crabitat by adding a lot of substrate to fill in low areas. It ended up being a whole new layer, to make it deeper for the crabs. I suppose that could have caused stress, but the crabs (2 med PP) have acted fine. Does an aggressive crab ever pull limbs off another crab? Last night I did observe the crabs interacting in a way I hadn't before. It seemed like one rushed up to the other and they kinda moved their feelers rapidly in each other's faces. At the time I wondered if they were fighting. Can plastic plants or climbing "toys" hurt their limbs?
Since one limb is lost, whatever the reason, should I expect more losses and possibly death? Can anything be done? FYI tank stats: 15 gallon, Substrate (EE mixed with Hermit Crab Beach, with a higher concentration of EE) 7 inches, temp 80, humidity 75, several extra shells. I also just switched from one brand of aquarium salts to Instant Ocean. ???? Thank you!

Is one of your crabs missing a leg? Is the found leg hollow, like just exo, or solid like a whole leg?

Hmm, good point. I haven't been able to check closely but it appears they both have all their front legs intact. Also, they have both been active, not buried, for a couple months so I don't think it's from a molt...
I'm pretty sure about 6 months ago our third crab died (disappeared), but based on the color of the leg (looks fresh) I don't think it's from that one.
Thanks for replying.