Why do some hermit crabs go naked?

[BlazeTheWeirdo]

Today I went to a pet store and took a look at their hermit crabs. The set up was of course, not proper, which was expected. In the enclosure, there was a dead hermit crab, naked, lying on the substrate (mixture of sand and soil).
All the crabs in that enclosure had painted shells so I assume his was too.

As sad as this is, it made me wonder, what are the reasons/causes a hermit crab will go naked?
I've been trying to research this topic for a while and so far I haven't found too many detailed articles on it.
So what are the reasons for a hermit crab to do this? What causes this phenomenon?

[Hermiesguardian]

One of the reasons is they are too weak to carry their shell. Or maybe if there's something in the shell bothering them but they haven't found a new suitable shell. Another crab may have stolen its shell and it was hurt or killed or too weak to get another shell.