King of The Hill is a type of gameplay where your objective is to control a defined section of the map while not allowing your enemy to overtake and control said section themselves.
IIn Overwatch, King of the Hill is called ControllControl. This is due to the style that blizzardBlizzard uses, if you watch. If you've watched some of their most recent Developer Updates, you'll hear Jeff say "we at Blizzard like to call things a little differently" While talking about their "Capture the Flag" game mode, which they called "Capture the Rooster".
Most content creators are old school gammersgamers and are very familiar with the "King of the Hill" games, that's. That's why this term is so widely used, while referring to the Control game mode.