By embracing the NAFTA treaty, Mexico surrendered its corn output to the vagaries of the North American corn market. Now, all of a sudden, Americans want corn, lots of it. As part of a dubious strategy to make “cleaner” gasoline while rewarding Big Agriculture and farm states, the U.S. Government is shoveling huge subsidies to corn-based ethanol production. This new demand has bid up prices in the U.S. and caused acute corn shortages in Mexico, which in turn has tripled and quadrupled the price of tortillas.
Which just goes to show that "free" trade is neither free as in freedom nor free as in beer.