There are a number of reasons why the cops may pull you over. You may be swerving, speeding, not following traffic lights and/or signs, or one of your lights are off. They may run your tags to see if it is a stolen vehicle. If the information is missing on there, then they may also pull you over for that. In addition they may also pull you over when they feel like it, and make an excuse on why they are pulling you over. It is up their discretion.
It really depends on what mood they are in. Some people say cops in certain areas like to racially profile. This means that when they see a minority such as a black person in an expensive vehicle, they will pull them over thinking they are dealers of some sort.