How did the word " Okie" devolve into a a derogatory term?

While washing themselves during a stop, the Joads encounter migrant workers who want to turn back. They tell them that the Californians hate the migrant workers. A good deal of the land is owned by the Land and Cattle Company that leaves the land largely untouched. Sheriffs push around migrant workers, whom they derisively call "Okies."