Hampton seemed to be stunned that he was unblocked and even more surprised that he was actually faster than Manning.
Hampton's job is usually to occupy the OL and let other people chase the QB, it's pretty rare for him to find himself behind the OL, I'm sure it was like finding a room in his house he'd never been in before. And seeing him catching up to Manning was pretty sad, Manning definitely needs to take some lessons on running, you have to be able to out run a nosetackle.