Urchin died

[Reef Pilot]

I had a Tuxedo Urchin die and just wondering what may have caused that. I have other urchins and they all seem to be OK. I thought there was something wrong with him the last few days, as he wasn't moving much and just sat in one corner of the tank. His spines would still move around, so I don't think he was dead, but it didn't look right. Then this morning, he was all white, and I removed him with a net. His spines were all down and broken off, too. What surprised me also, is that he was completely hollow inside, like something ate his insides.

I did see a big hermit crab hanging around him the last couple days, so am not sure if he had anything to do with it.

Any ideas what may have caused my urchin to die?