What was the major cause of the east st. louis riot in 1917?

Job competition and racial tension. 

This riot occurred in the midst of World War I. Factories were in need of workers and blacks filled the jobs. Whites were worried about this and attack blacks in violent riots. 

Blacks arrived to the North seeking industrial jobs in a huge movement called the Great Migration. Blacks kept factories producing while whites were in Europe fighting World War I. Remaining white men were worried about racial imbalance and lashed out. The East St. Louis Riots are considered the worst work-related racially motivated riots in US History. 40 blacks were killed and much property was destroyed as mobs marched through the city.