Two sudden deaths

Of the 6 crabs I got in september I've had 1 die due to a failed molt and two suddenly unresponsive and swarmed by springtails on land.

Both I had seen earlier that day alive and moving but dead later.

I saw 2 of my other crabs alive and moving and I'm getting stressed and depressed what is causing it, they haven't bred either and I can only hope that they died of old age, but I wonder what the chances are buying from captive raised.