A wtiter named Matt Schafer wrote a book called the "Mandinka Legacy in the New World." In the book Schafer wrote that he believed that Southern accent is from the Mandingo slaves that were brought to the USA.
The Mandingos were some of the earliest people brought to the USA by the English and the Southern accent likely developed in South Carolina and Georgia were the Mandingos were heavily enslaved. Schafer also traveled to West Africa to listen to the Mandingos speaking English and he noted that they spoke English with what he believed was with a thick Southern accent.