I know nothing about cop procedures, but common sense tells me if they're going to put handcuffs on a guy, the place NOT to do that is at the driver's door, with it open, and otherwise ready to roll.
Get him away from the door at least and CLOSE IT.
As I understand it, the perp was stopped on a normal traffic stop for whatever reason. When they ran his name, they found out he had an outstanding warrant, which is why they attempted to arrest him. They weren't "serving a warrant" as I've read recently.