Many animals are capable of entering a trance-like state called tonic immobility whereby they appear dead to their surroundings. In the case of sharks it has been observed on many different species such as the lemon shark, reef shark and tiger sharks upon simply placing them upside down.
During tonic immobility the dorsal fin becomes straightened and the breathing and muscle contractions become more relaxed. It is such a reliable behaviour in certain sharks that it is used as a type of anaesthesia before minor surgery.
Some killer whales have learned to take advantage of this by using their tails to create currents in the water that can turn a shark over in order to eat it. The reason this happens to sharks is unclear, but it can be argued that tonic immobility has a role to play in survival allowing the shark to blend into the surroundings by being completely motionless, but in this case it’s obviously a disadvantage for the shark.
It has also been speculated that it may be something to do with the mating ritual of certain shark species as in some cases it can be induced by massage.