Here is a really interesting article (in Spanish) by Jéssica Nájera Aguirre from the Colegio de México on circular migration between Guatemala and Chiapas, sometimes on a daily basis, always with the intention of maintaining residence in Guatemala. We're talking 300,000-400,000 people a year.
What I don't get, however, is how there is so much demand for labor when--at least on the surface--we might expect plenty of Mexican supply already to exist.
Given the high levels of poverty in Chiapas, is there not resentment on the part of Mexicans toward Guatemalans who cross the border and sell goods (which is the second largest source of employment)? Further, why is it that agricultural demand is not being met with local supply?
I don't know the answer to those questions, but what we see is a border region that is now transnational to a much greater degree than in the past. I imagine the reasons for this can be in found to a significant degree in Guatemala. The article does not address the issue directly, but clearly Guatemalans cross the border because opportunities at home are too scarce, and in recent years--more or less the last decade--the sense of insecurity has increased. And as we know from the U.S. side, once this type of migratory pattern starts, it does not stop.