Accelerating while overtaking

I really don't understand what sort of "so called driver" you have to be to do this kind of moves. Since it's a rather long video, it has the highlights at the start, and the whole interaction later.
From what you can see, this Audi driver was holding up another Audi even before I came into the picture, braking for no reason, making others behind brake as well. I wasn't gonna push it since it was wet, gave both of them quite a bit of distance, but I knew there was a stretch of straigh road ahead, and waited for my chance (mind you, I have a rather underpowered car).
First Audi, not a problem, but decided to wait for the next one because of oncoming traffic. It probably is a good idea that I did wait because what happened afterwards could've been lethal if he'd done it with oncoming traffic.
The Audi I tried to overtake next was not having it! Mid overtake, he accelerated, so I didn't stand a chance, and yet seconds later started braking for no reason on a straight stretch of road. Dispicable!
I saw what kind of person I was dealing with and decided to back off although he was driving below the speed I would usually drive.
I'm not into playing these silly games. I had my tempo before I came across him, and all I want is to resume that tempo. It's these kind of people that are hideous and making these roads unsafe. And for what??? Absolutely no reason! After all, watch the whole clip, and explain, what the hell was he thinking????