What was the major cause of african american migration from the rural south to northern cities during world war 1?

Jackson pollock described his art style in the following quote: “new needs need new techniques.… it seems to me that the modern painter cannot express this age, the airplane, the atom bomb, the radio, in the old forms of the renaissance or of any other past culture. each age finds its own technique.” what point is pollock making about the new styles of art?