The problems faced by the Mexicans are primarily economic and they all have been coming to the USA in search of cheap labor.
America offers them cheap jobs and a spare of donate used.
Many Mexicans work as laborers, small time shopkeepers, and help set up small stalls near the Mexican borders, offering a good market to the Americans.
Many Americans also encourage migration.
Many of these Mexicans run their smuggling cartels and carry guns.
They bring low level conflict and violence to the American society and have their own ghettos.
A whole lot of Americans, especially the Whites, are opposed to them.