Could be many reasons. Normally its just to get the plate number to run it thru the system but sometimes it feels like they are trying to scare or intimidate you to do something wrong. I ride a cbr1000 and have cops tail me all the time..I removed my mirrors because of this. Before I would notice them riding up so close on my ass I couldnt help but speed up for my own safety, or I would stare at them in my mirrors instead of watching the road. When im in a car I just brake check them now because its really unsafe for everyone when cops think they are racecar drivers and get 2 inches away from your rear bumper.