thats because angles dont nip their winfs they will pick there eyes out or cause them soo much stress they die as in your case.
never keep angels, puffers or triggers with sharks or rays
im not jumping on you but it doesnt sound like you did any research before buying the ray, also what kind of ray was it? it could have been a cold water species and you temp could have been to high etc.? that would easily cause a baby ray to stress and die
the crab "jumped" him because he wwas already dead behind the rocks, crabs eat dead thinggs.. rays are way to fast to be caught by a crab. i used to have a strawberry hermit the size of a baseball with my rays and they would never let him close to them and beleive me if any crab could catch a ray it would be a strawberry hermit that big and he had NO CHANCE
either way ims orry for your loss,, just do more research next time