How did the expansion of slavery to the west cause conflict between the North
and the South?

The North, especially, was afraid that the South would force its “peculiar institution” upon the entire Union. These fears were realized when the expansion of slavery into western territories entered Congressional debates.

Explanation:

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.