Why do sharks eat whales?

All the whales usually eat the sharks as their normal food diet. They also even eat the killer whale or the orca.

Ocean Food Chain:

Sun>Seaweed>Zooplankton>Plankton>Krill>Shrimp>Small Fish>Medium Fish>Big Fish>Shark>Whale

The probable reason why whales might eat sharks is to protect humans and help reduce the shark attack risk vulnerability.