Cops may have overreacted to his delay in pulling over, but he then began disregarding simple instructions. For all the driver knew, he had been mistakenly identified as a murderer. Hunkering down in your car isn't going to be the healthiest option under those circumstances.
He was ordered to keep his hands out the window and to exit the vehicle. Since he was wearing a seatbelt, I don’t see how both were possible. At that point, reaching down to unbuckle his seatbelt would have almost surely resulted in him being shot.
