Watching the game live, the home broadcast team showed quite a few replays to try and get the TMO's attention. The side on view that has been played most since doesn't look great. The other angles show it in a better light, or so I thought at 2am. He looked to be making some token effort to bring his arms up and I'm not sure there was contact with the head, if it was it was very marginal. I agree it was a poor tackle and he is actually lucky to not get a few weeks but I'm also not surprised that he got off with a warning. He doesn't usually do stuff like that so I feel it was out of character.
What brought it and the other dodgy shot earlier in the game on? I have no idea, but I wouldn't mind guessing that he was just pumped for his good mate and mentor's 100th cap and really wanted to get out there and put in a massive game for him. The blood went to his head and that was the result. As I said though, just guessing.