This is purely my speculation.
According to Toyota, Mr. Sikes pressed his brake pedal hundreds of times during his "runaway Prius" event, but did not trigger the engine torque over-ride.
One possible explanation is that perhaps he had a foreign object, such as a soda bottle, wedged under his brake pedal. In a panic and unable to slow his car down, he wasn't able to remove it until the end.
I once had this actually happen to me, when a water bottle from under my seat worked its way forward, and lodged itself under my brake pedal. It was a little hairy, but I was able to kick it out of there before getting in trouble.
Giving Sikes this bit of leeway, we're still stuck with the other problem: how did the Prius run away to begin with? According to NHTSA and Toyota, his floor mat was not clipped to the floor, but was not bunched up under the throttle pedal, either. But they were unable to move the mat into a position that would have pinned the pedal. And his Prius was not subject to the throttle pedal recall.
Did Sikes get a stuck throttle and an obstructed brake pedal? Or did he make the whole thing up for publicity?