Snails can die for many reasons.
Salt[]
Snails can die due to salt. If you put salt on snails. The skin of it's body will burn off. It can be used to kill snails to make escargot so you don't have to salt it after it's cooked.
Shell Crack[]
Inside a snail's shell is it's internal organs. If you crack it's shell. Then it's circulatory system leaks out.
Old age[]
Everything dies this way. You might be healthy at age 105 but you can die the next day.
Humans[]
Humans kill snails for just the fun of it(sick humor) or for no reason at all. Sometimes it may be an accident.