It was at most a mid strength shoulder barge. I don't think for a moment Agbonlahor would have wanted to seriously hurt him.
It wasn't anything dangerous, just totally unnecessary. Look at his snide little smile after it happens too, pretty sure he knew what he was doing.
That said, 1D dude lost all sympathy with that hair flip while getting up
Of course he knew what he was doing, he was grinning as he did it, it was clearly just meant to be a joke.
How was he to know the 1D bloke is in fact a gigantic vagina with a shit haircut?