Why was President Hoover's Mexican Repatriation Program legally questionable?

Around two million Mexicans and Mexican-Americans were deported between 1929 and 1936. The issue is that most deportations were not legal, were made under the threat of deportation or under the threat of raids by local police in many states, or there were physical deportations outrightly by local authorities withour meeting law requirements. Great injustice was committed, because it is estimated that 60% of those people arrested in clandestine raids and deported were either US permanent residents or citizens. The program fed xenophobia and strong anti-immigrant feelings in the hard times of the Great Depression.