I think the reason for this transfer of residents might be caused by Dallas, Charlotte and Atlanta's favorable conditions of the black populations. They might have felt that they are better off living in this area. There must have been better laws that provide benefits and opportunities. These areas in the US have a big population of black people it helps them agree with each other.
The idea and concept of Popular Sovereignty is the belief that the authority, legality and legitimacy of the government is created by the will or consent of its people.
