That would be the North American Free Trade Agreement.
KHOU:
“Jose” came to this country 10 years ago, seeking a better life. Now he is headed home.
Since he does not have a valid Social Security number and recent government crackdowns, employers who in the past would have hired an undocumented worker are too scared.
[…]
Officials at the Mexican Consulate’s office in Houston said more families are headed south of the border. The number of Houston families transferring their kids’ transcripts into Mexican schools has more than doubled.
An immigration attorney blames tighter enforcement for the migration, but that is not the only factor. The economy is improving to the south as companies are expanding into new global markets.
http://www.landlinemag.com/Special_Reports/2008/Aug08/080408_Cross_border_update.htm