From stress actually. Its common for captivity crabs. In the wild they got stress from the competition of other crabs, in captivity, they build up stress from bad tank environment, low humidity, too high / low temperature, lack of shells.

Lil buddy passed away around 9pm. I let it rest in the main tank and died peacefully between the stack of shells. I removed the table lamp from the tank, so none of my family members will switch that on accidentally.