Barrett was pissed off from kick off.
PJ Marsh obviously went to charge down the kick. You can look at it if he was there half a second earlier, he would have successfully charged it down as he would have made contact with the ball.
Marsh didn't go high at all. And if Barrett had any brains he would have stayed on the floor, milked the penalty for St George and instead got momentum for St George. Instead, he gets up and king hits Marsh from behind. Look I've got a lot of time for Dragons fans and I say it frequently that they are more knowledgable in the league but the way Barrett carried on was just rediculous.
But as both players involved have said, this sort of thing happens and you just leave it on the field. Besides Barrett was booed everytime he touched the ball and at one point in the game there was a very loud "Barretts a wanker" chant going.