I'm under the impression that a drip kick in the field of play has to be a goal attempt. He clearly didn't go for goal, so he just messed his timing up due to the pressure from Tomkins, so knock on.
Anyone that thinks a player purposely attempts to do a drop-grubber is clutching at straws. From a technical coaching point of view, to attempt to hit the ball on the half volley is a lot harder to control than straight out the boot, so it is a ridiculous practice.
And Tomkins had every right to challenge. He was committed to the tackle. McGuire had the ball in hands when he made the attempt. What was he meant to do? Leave him? He could have dummied.
It was a clear knock on. And if any similar incidents have been given in the past, then they were wrong. Doesn't make this any more right!