Why did thousands of African Americans migrate north during World War I?

At that time, the South was still in the grip of the so-called Jim Crow laws that ruthlessly enforced racial segregation. African Americans were subject to widespread discrimination in all aspects of public life, including housing, employment, and education. The authorities, both legal and political, treated them as second-class citizens, denying many the right to vote or to run for public office by way of cynical manipulation of the law. To make matters worse, acts of extra-judicial murder, or lynchings, were disturbingly common, especially after the rebirth of the white supremacist Ku Klux Klan in 1915.
Given such a toxic environment, then, it's not surprising that many thousands of African Americans headed north for a better life. At that time, the heavily-industrialized northern economy was growing rapidly, creating new job opportunities. After the United States entered World War I, the booming armaments industry created even more, and increasing numbers of African Americans entered the workforce. Although the North still had more than its fair share of racism and discrimination, its quality of life was a world away from what it had been down south.