Based on an accumulation of evidence, we now know that the government's action was partially initiated by California corporate agribusiness interests hoping to satisfy their own lust for land while ridding themselves of competition from the state's most productive family farms.Wikipedia adds:
Japanese-American farmers were a huge presence on the pre-war West Coast, producing more than 40 percent of California's commercial vegetable crop alone. A June 1942 federal report noted that "the Japanese people were the most important racial minority group engaged in agriculture in the Pacific Coast region. Their systems of farming, types of crops and land tenure conditions were such that their replacement by other farmers would be extremely difficult . . . . The average value per acre of all West Coast farms in 1940 was $37.94, whereas that of Japanese farms was $279.96
Incarceration of Japanese Americans, who provided critical agricultural labor on the West Coast, created a labor shortage, which was exacerbated by the induction of many American laborers into the Armed Forces. This vacuum precipitated a mass immigration of Mexican workers into the United States to fill these jobs, under the banner of what became known as the Bracero Program. Many Japanese internees were temporarily released from their camps – for instance, to harvest Western beet crops – to address this wartime labor shortage.