Monetarist theory is a theory of economics proposed by Milton Freedman which asserts that Keynsian economics only applies in the limited case that central bank need to keep the money supply growing; otherwise, the free market can handle itself.
Therefore the Monetarist theorists propose that the stock market crash of 1929 was caused that the US monetary fund did a bad job of actually controlling the funds, and didn’t inject enough money into economy.
See also the opposite: demand-driven theory.