They're looking for anything that sticks out. It could be...

They're looking for anything that sticks out. It could be someone driving too slow, or driving too fast, ... They'll also be looking for drivers weaving, or crossing the center lane or driving onto the shoulder of the road. Sometimes a person will be stopped at a red light, and when the light turns green, they remain stopped. Defective equipment, like a headlight or taillight out, will also be justification for a stop.