I don't think those trade agreements are the main reason.

I worked for a law enforcement agency in San Diego County in California in the late sixties and early seventies. We had a standing invitation to assist the US Border Patrol when we were off duty. We would wear our uniform shirts and badges, and Levis and boots, and ride shotgun with a BP officer in their 4x4 vehicle.

On weekday nights, just in the area East and West of the San Ysidro border crossing, we would CATCH over 2000 illegals each night. On the weekends, it was over 3000 per night. And that's just the ones we caught, which I'm sure were just a percentage of the total that came across.

And none of those trade agreements existed at that time. In fact, during the first part of that era, the Bracero program was still in effect.