Or he could be looking for a DUI arrest. Drunk drivers (or tired drivers) will do all sorts of things that call attention to themselves, including driving slowly or braking suddenly when there is no obvious reason to brake. It happens to me when I am tired. I sometimes think I see something running across the road and it causes me to want to brake. It usually scares the piss out of me and gives me a nice adrenaline high for the rest of the ride home.
Was the cop a dick or unprofessional to you? Do you know why he pulled over the other car? It is his job to pull over cars and, if he has a legitimate reason to do so, whats the problem? Perhaps there have been complaints about drunk drivers leaving the pub, or there have been accidents. My department does not prohibit hanging out near bars to pop DUI's, but I have never known anyone to do that.
I cant say he was not, as you put it, "Power hungry" but he sounds like he was out doing his job and doing proactive police work instead of just sitting around waiting for an accident call.