Workers in the 1930s faced the economic realities of the Great Depression and 25% unemployment. Workers were expendable - easily replaced by any of the other hundreds of thousands of Americans looking for work. So wages went down, it was easier to get fired, work was more temporary in nature and had none of the insurance or benefits we have come to expect today.
Migrant agricultural workers came, not from foreign countries, but from economically devastated US states like Oklahoma and Arkansas. The migrants were white families looking for any way to make ends meet. Pay was low, working conditions were sometimes dangerous or difficult, and workers had no way to pressure their employers to improve those conditions.