Its easy. In the 1920s and 1930s, blacks moved to the big cities from the miserable crapholes that the Democrats created for them in the South. Now they are moving out of the miserable crapholes that the Democrats created for them in the big cities. Name one place that is controlled by Dems that is not economically dying.
Most of the blacks who moved from the South were both escaping segregation and worked hard to create a better life for their families. Once a couple of generations passed and/or the Great Society and liberal policies destroyed the black families the areas where they lived declined precipitously.
Enter now the illegals who are making life rough for blacks in the hellholes they live in because the illegals are worse and are not afraid of being punished for acts against blacks so the blacks are leaving.
Once the family unit is destroyed, religion (moral) teachings are removed from a society, and law and order breaks down chaos reigns. We are at the beginning of the chaos stage.
Trump is the only thing between us and total breakdown.
Kudos, the most succinctly factual explanation at post #2.