Most likely the snails were weakened, and probably would have died anyway. First, the snails may not have enough of the right type of algae to eat...there may be plenty in the tank, but what they need nutritionally may be in short supply...possibly there is a change in water chemistry- turbo's are far more susceptible to chemistry changes than hermits or fish. They could have fallen down- turbo's and astreas have trouble righting them selves when they fall on their backs and there's nothing to grab on to. When a hermit sees a snail in this position, they go for it. Sometimes the crab inadvertantly rights the snail, in which case the snail is fine. If the snail weakens, the crab wins. Take care, Eric