Mexico is a Marxist state that has persecuted the Church bitterly for about a century. Mexicans know little formal religion, and if I recall correctly, Mexican immigrants have a remarkably high proportion of leaving the Church.
Motive is the same as all the other leftists: bring like-thinking illegals in to dilute and undermine the “devil” right wing America.
Take a look at what's happening in Mexico:
http://www.rnw.nl/english/article/islam-new-religion-rebellious-mexican-state-chiapas