We didn't see the same video. The cop is apparently behaving politely but the tone of his voice indicates a bit of an attitude. Like he is pretending to be polite but, at the same time, trying to establish who is in charge. The driver doesn't like what the police is saying, tries to question his actions, and must also have shown some kind of attitude right before the cop said 'why so upset'. At this point, I imagine the cop must be thinking 'this guy is hiding something and is going to resist', and the driver is thinking 'this guy is an asshole and is stopping me because I'm black'.
Then, the cop tried to grab the drivers hand probably to handcuff him, the driver resisted, and there we go!
Obviously the cops used much more violence than needed. Even if the driver was resisting, he wasn't a menace that deserved being punched in the face so many times. The cop had him under control when he was on top of him. At that point I think the cop lost it because he was pissed off. And he must have been thinking 'OK, if you won't stop resisting, I will beat you up until you stop'.
That's why I said resisting never makes sense. If you resist, then they can kick your ass. If you don't resist and they kick your ass, they are in trouble.