More than likely the shrimp dies during acclimation, they are very sensitive to it. I remember I lost 6 cleaners in a week due to a bad method when I started.
Crabs are scavangers and oppertunists, more than likely they found a free meal.
Are you sure it was a dead shrimp, and not a molt? jsut had to ask as I have had a few people bring in molts and said their shrimp died.
Ken