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.
So you say, "I have my seat belt on ... Do you want me to remove my hands from the window to unfasten them?"
Hard to have a conversation with two pricks pointing guns and screaming at you.
