Maybe I haven't read Rise of the Horde too recently, but I recall it was a point where the Gronn were definitely beating the Ogres and the Ogres preferred the treatment the Orcs were offering them. It's also a point that Ogres do respond to violence and strength. He at least needed to prove to them his Horde was strong, because it being preferable to the Gronn wouldn't have been enough on its own if they thought the Horde was weak,