Migrant workers migrated to follow the harvest, which comes at different times at different latitudes. That was the migration part, not storming our borders. Those migrant workers who were not US citizens were supposed to have green cards enabling them to work (and often did around here).
The problem is one of popular nomenclature, allowing the media to refer to illegal aliens as "migrants". (Now, virtually anyone who breaks a sweat who is here illegally can be referred to as a "migrant worker", whether they migrate to keep up with crops ripening or not. That's abuse of the language.)