High g's at re-entry. Why?

Hi all!

It is said that the SpaceShipOne experience the highest g's on its return down to the rest of us.
Why is that? Is it the decceleration because of thickness of the atmosphere?

Yeah basically. After he starts to fall back to earth he gets going quite fast. So you have heard that at 40 miles an hour water feels like cement... well same thing, except its air. He is being slowed back down below/to terminal velocity and rather quickly.

This why you get heating while coming from space. If you had enough fuel you could just fire up your rocket engine and land it as easy as you took off... Buck Rogers style.