Why do water buffalos eat hippos?

The water buffalo is a very important animal in Asian and European agriculture. A lot of Italian Mozzarella is made from buffalo milk. It's a grazing animal and a vegetarian except when it inadvertantly ingests bugs and worms in its food. The concept of a water buffalo eating a hippopotamus is a most unlikely, not to say impossible, one. Even if a water buffalo should show such carnivorous ambition the hippo would be more than capable of resisting being eaten. Not a lot of creatures eat hippos, even those who live on the same continent - which water buffalos don't, in the wild.