Thomas Sowell has repeatedly made the case that government intervention is largely responsible for the Great Depression. For example, he notes what happened with the unemployment rate in the months after the stock market crash of 1929. Based on Out of Work by Vedder and Gallaway (alternatively available through JSTOR), page 77, one sees that the unemployment rate went from 5% (Nov. 1929) to 9% (Dec. 1929) and down to 6.3% (June 1930). It is then that Hoover signs the Smoot-Hawley Tariff Act, which Sowell contends is the first cause of the Depression. He then notes that by December 1930, the unemployment rate had shot up to 14.4%. Furthermore, aside from minor fluctuations in early 1931, the unemployment rate dips just below that 14.4% figure in 1936, but that is still the unemployment rate in December 1939.
I'm wondering what alternative factors or explanations could explain the quick increase to the unemployment rate followed by a decrease that approaches the previous numbers and then the marked and sustained increase, or if such alternatives even exist.