Many states have now set up DUI Checkpoints. These are not targeted based on behavior: Every car traveling between point A and point B is expected to pull over and take a breathalyzer. These are often targeted based on neighborhoods or proximity to 'party' districts. They are conducted without warrants, and cast a wide net on individuals traveling.
I see. Okay, I didn't know that happened. I've only ran into one such checkpoint in my life and the policeman just asked me where I was coming from and where I was going, asked if I had anything to drink while pointing his flashlight into my car; I assume looking for an open can/bottle or anything suspicious. That last like 4 seconds, then I was on my way.
I'm going to guess you're white and probably look reasonably mainstream. Somebody more "suspicious"-looking will often end up with a less smooth experience.
