what were the three reasons w. e.b dubois suggests that motivated african americans to leave the south during the great migration?

African-American leader W.E.B. DuBois noted that the first cause of migration was economic crisis.  Flooding and boll weevil infestations in Alabama and Mississippi had devastated farm work there.  Occurring at the same time, immigration from Europe to the United States had been curtailed.  So that meant there was a strong demand for more workers to move to the industrial northern states.  Thirdly, there had been outbreaks of violence against blacks in the South, notably in Georgia and South Carolina.

Moving across the country (or to a new country) is a hard decision to make.  If life is bearable where people are living, they're likely to stay there and endure it.  But when factors make leaving less risky or painful than staying, that's when large migrations of population will occur.