What were some of the root causes for prejudice against the Okies during the Great Depression?

After the Dust Bowl in the Panhandle,Colorado,Texas, and New Mexico over 1,000,000 people from the area moved to California causing overcrowding, lack of jobs, and discrimination. The school teachers often ignored the Okie students, shoppes posted signs stating (Okies cannot shoppe hear go home), and jobs posted no jobs keep on driving.