He almost looks like he singled her out the way he does that u-turn and follows her.
Could be. A cop friend of mine once told me that you can find a reason to pull pretty much anyone over. People are typically bad drivers and will miss important things. I can definitely confirm that something like half of drivers I see don't actually use their blinkers at all. Follow somebody for a little while and they'll roll through a stop sign or turn without a blinker or screw up right of way at an intersection. I know sometimes cops will some times react if you look at them like you're afraid of them or you have an issue with them. Given the whole not putting out her cigarette thing, maybe she scowled at him or something. Not to say that's justifiable, but it could be he assumed that because she doesn't like cops he can find some drugs or something. Or, you know, he's just racist.
In some jurisdictions you must signal a turn 10 seconds before you make a lane change. The Idaho cops pick of Oregonians left and right every time there is a music festival out there. Of course the pull over is just there to create a pretense for a drug search.