Alistar can Headbutt Sion during his ultimate

At the end of a game, as Sion I ulted towards the enemy right outside our base. As I was running towards them, Alistar knocked me away with Headbutt. I was knocked back and briefly stunned even though Sion is immune to CC during his ultimate. This was the only time it happened during this game. I'm not sure if Alistar tried to do this earlier in the game; also, I have not tried to reproduce this since I don't play LoL with friends. Here is the match link for that particular game.