Great migration was a northern migration. It was the movement which was before Africans and Americans to move out of rural southern part of United States to the urban midwest, Northwest and West.
The main cause of Great migration was for black southerners who wanted to escape segregation which was referred as euphemistically. They believed that in the North there were less prejudice, racism against blacks.
World war caused shortage of labour in the factories which were in the North. Americans and Africans migrated to take advantage of education, modern facilities and job opportunities.