snails do some strange things.... and i think it's just because they know it drives us bonkers, lol.
snails quite often will hide in their shells if there is something in the water that is irritating them. what is your water change/maintenance schedule on this tank?
they'll also hide if there are fish in the tank that are harassing them. what are the tankmates?
if you ever have snails float or stay in their shells for an extended period of time (a week or more), do the smell test. trust me, you'll know if they are no longer living.....