[Michael Hoaster]

That's true, dinos can kill snails, depending on which dinos they are, but I don't think those are dinos. I think they are vermetid snail tubes.

Snails are sensitive to water chemistry changes. A water change, some chemical introduced on your hands or a cleaner used nearby could be a culprit. Also large snails like that need a large tank to support enough algae growth to keep them well fed. It may be that it has exhausted it's preferred algae and is eating whatever's left, but it's not sustaining it. You might try adding more vegetable matter. Or, as teddscau said, it could be old age. Five+ years sounds pretty old, but I have no idea of their life expectancy. These are just general ideas. You might try googling up some research specific to them to see what you can find.