The Great Depression happened because
after the 1929 stock market crash, which was brought about by a combination of radical margin requirement tightening in the days preceding it, an increase in interest
rates that further dried up the cash that was being used to buy stocks, reaction to the floor vote reporting on the Smoot - Hawley tariff bill (which made it clear it would pass), and a
concerted selling / manipulation effort by Wall Street's biggest players, the economy was in shock.