There were many reasons but most analysts agree that it was triggered by the stock market crash of October 24, 1929 also known as "Black Thursday".
 

Other causes  Overproduction  factories had previously produced too many goods which now could not be sold because there was no demand for these goods. To protect their markets countries began to raise tariffs. Canadian industries, such as mining, wheat farming and pulp and paper, were especially hard hit since they depended on export sales to the United States. International trade slowed down.

• A vicious cycle began, forcing companies to lay off employees, cut back on production or go bankrupt. This further decreased demand for goods. Prices and wages dropped.

• In Canada, the Prairies were hit with a drought in 1930, which destroyed the wheat crop for several years (the Dust Bowl).