I don't know about yours, but I had a nerite snail that started doing that one day. A few days later, my RCS had started to pick away at it (it had died). I think it just died from old age. I'd say just keep an eye on it.
Before this all happened, I came into work to find that the nerite had climbed out of the tank and traveled 3ft away before rolling back on it's shell. I thought it was dead, but I decided to put it back in the water just to see, and after an hour or so he was back to normal.