Hermit Crab Random Death (not the mispost)?

Random fatalities do happen, sometimes it is the stress hermies go through by the time they reach pet stores.
Hermit crabs are not hard to keep, but their keeping is very strict, Too hot, they die, too cold, they die, not the right humidity, they die. They need dechlorinated plain drinking water as well as a salt water bath.
The substrate must allow them to dig, destressing or molting usually. Many small things to keep them from stress and keeping them healthy.
Yes, shell fights are possible and so is cannibalism under certain conditions. Streaking hermies are not uncommon,but the point of their shell is protecting their delicate abdomen.They are very vulnerable out of a shell.
What may have brought this on is very hard to say, If your crab keeping is proper, they have a roomy tank, things to climb on and company of others, shell fighting is likely, the larger just shaking the smaller from its shell.
Where they sell hermit crabs, they may sell a book called "Hermit Crabs: A Complete Pet Owner's Manual", by Sue Fox
Worth the read for a few dollars. A couple of keeping sites/forums are a good pace to read about their keep:
https://www.hermit-crabs.com/care.html
http://www.hermitcrabassociation.com/phpBB/index.php
We had 5 hermit crabs in a 35 gallon tank that lived over 10 years,and properly kept, should live that long if not more.
Consider, hermit crabs are not bred in captivity, they are removed from the wild,and that begins their heavy stress until properly kept. Better luck with future crabs.