You can read more details here.
It appears Kelly was mad because the referees didn't want to chat with him about a call they'd made.
“I wanted to get an explanation from the call, and he wouldn’t come talk to me,” Kelly told the Eugene Register Guard. “That was what I was mad about. And I wasn’t mad, I just wanted an explanation on what the call was. That was the only concern I had.
Of course, even more entertaining was Kelly telling a group of rowdy Oregon fans after the game to "show some respect" before he did post-game interview with Andrews.