Some invertebrates are extremely sensitive to environmental changes. It could have been something very small. Also possible that he was simply an adult shrimp and was getting old.
All that being said, I doubt two hermit crabs that size could take down a skunk cleaner shrimp unless it was already near death. At worst, they ended the struggle for him.
And though it looks solid in the picture, are you certain it's not a moult?
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef