Feather dusters can die of stress.
A friend of mine had a cleaner wrasse that used to bust into and wallow into his prized
feather duster like a clown in an anemone. It would loose its crown often, then up and jumped out of the tube and died one day for no reason. All water params were fine.
Often they bail out of their tubes for no reason at all. Usually if they can get their backside covered they will regrow the tube and be just fine. They may not have liked the amount of light or current they were in. If they cant get themselves buried or protected they will get chewed on by crabs, fish, etc. Especially hermits, they LOOOOOVE feather duster (scarlets, redlegs. bluelegs, whatever).
Of course I imagine they die of old age too. But I'd suspect something in your tank environment may have caused the problem. If your water params are perfect.... did you have a fish playing with them? Too much current perhaps? Too much light?
