He allegedly saw the man he thought was Newell enter the vehicle. Then he ran the plate.
Did he run the plates before or after he pulled the driver (who was not a black man) over? Did the driver commit some traffic violation? Was it just 'suspicious' that a white (at least she looked white to me) woman was driving a car with a black male passenger? Now, 40 years ago, that would have been unusual. It isn't anymore, even in the 93% white, <2.0% black community where I live.
Please read my post above. I think it helps explain what happened, and it may be very a case of reasonable mistaken identity that was not handled as well as it could have.
One problem I have is that the woman in the vehicle was charged for this incident, which seems absurd based on the control the man in the car was asserting.