Seriously though it's a number of things to choose from. In general, law enforcement officers are just dickheads. They deal with constant bullshit on a daily basis, they are tasked with being on the front line with the mental health crises (that is much more than just old people with dementia) and many many of them don't have the psyche it takes to be effective at the job because the pay isn't there.
He could have been working on a traffic safety grant where he must make two contacts per hour and hadn't seen anyone in a while. Told you the radar said 78 because people rarely pay attention to how fast they were really going. It's an easy warning contact. You questioned, pissed him off, went from there.
He could have not even had a radar, guessed you were going faster than the speed limit and just threw a number out there. Checked you for warrants, seen you didn't have any and let you go.
Or he could have just been a real asshole who likes to fuck with people because he's a piece of shit as a human being and wants to be on the radio making traffic stops all night.
I'm the first person who will argue against a use of force claim of brutality because as bad as the state of law enforcement is in this country, I don't believe cops or deputies go around beating the shit out of people for no reason nor do I believe they murder people in cold blood just because they can. BUT, there are some real piece of shit human beings wearing a badge who are just that.
It's probably a good thing for you that you didn't decide to get arrested. He got you out of that car for a reason. He wanted you to be standing directly in front of that vehicle camera if you decided you wanted to curse or do anything else that would give him a lawful reason to bury your dick in the dirt and take you to jail.