I have never noticed that my red legs are doing this, but I have seen my blue legs do this on a few different occasions. That's not to say that my red legs haven't done this, but I have not seen them do it.
I agree with all of the above. Hermit Crabs are predatory omnivores with a fondness for fresh meat.
Typically though, snails can protect themselves for all but a gang of hermits. Were the snails weakened in any way? Perhaps osmotic shock from a rapid acclimation, copper from tap water, low alkalinity, overdose of Iodine, or starvation from a lack of Diatoms?