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.