To be perfectly honest, lots of shark researchers have similar pictures (as I am sure do long line fishermen). This sort of thing is not really that rare. I recall seeing one of a dogfish on the line, in the mouth of a sixgill, which itself was bitten in half by another (unknown) critter.
Long lines attract sharks, and sharks are not picky about going after the bait rather than critters which have already taken the bait and are hooked. It is often said the best way to catch sharks is to go fishing for salmon.