Actually, he did get bull rushed over. His pads were too high in his stance from the get go. That first burst from a defensive line is always going to be low to try and get below the o-linemans pads. Get them rocking backwards.
Even when you look at his stance before the snap, he looks to already be on the balls of his heels, which means he already has a backwards lean looking to "catch" the defender. That is fine, but you cannot be that upright.
That is exactly what happened and why he went down on his keister.