One thing I know from talking to the Border Patrol guys I work with is that the illegals meet up with transport once they cross the border. They're frequently driven by Americans, who get paid several hundred dollars to take them to cities. You were probably stopped to make sure you weren't one of those. I heard the story how one agent, who was off duty and in civvies, picked up a woman he had just seen leave a van by the side of the road. She claimed car trouble, but he knew she had just left a "load car" at a rendezvous point. Since he was off duty and didn't have any real proof anyway, he didn't arrest her, but he notified his BP colleagues of the drop point so they could wait and pounce when the illegals showed up. What was interesting to me was his use of the term "traitor" when he talked about her.