What was a cause of the economic prosperity in the 1920's?

There were a few reasons, part of it was because the European nations had just smashed each other -- and their economies -- to bits in World War I. The United States had undergone quite an industrial build-up during the Great War, and when the war ended the factories switched production to consumer goods like washing machines, cars, etc. That shift along with the rise of consumer credit created a higher standard of living for many Americans. This higher standard of living created enthusiasm in the strength of the economy as well, so many ordinary Americans started trading in the stock market which caused it to bubble.