What were the main causes of migration in the late eighteenth and early nineteenth centuries ?

The second significant cause of the Great Migration was the desire of black Southerners to escape segregation, known euphemistically as Jim Crow. Rural African American Southerners believed that segregation - and racism and prejudice against blacks - was significantly less intense in the North.Jul 2, 2016

the mongolian invasion of china and central asia resulted in a disruption of the silk road as the chinese garrisons lost support from the chinese kingdoms which fell under the mongols. however, as the mongols established a formal dynasty in china, the successors of genghis khan restored the garrisons along the silk road allowing the safe travel caravans again.