Why did hermit crabs die?

Two hermit crabs were purchased together from a pet store, one was large and the other was small. Both were provided with adequate food, water, habitat, extra shells, and temperature. One morning the larger one fell out of its shell dead when picked up to be misted, this was about two weeks after their purchase. The tail end of it was completely devoid of color and the crab was stiff and brittle. Two weeks later the smaller one's front leg and a piece of his larger claw fell off. The next day he too died. What could have caused their deaths?