1929 stock market crash and unemployment rate 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.
One caveat to the author you refer to, Thomas Sowell is a staunchly conservative economist, so most arguments you hear from him will stick to the Econ 101 wisdom that government intervention is usually bad, tariffs are always bad (to be fair, it is hard to use them to correct externalities for global "public goods"), etc.

Although tariffs probably were not necessarily the right choice for a tightening economy, the general story is that our central bank in America back then was a lot less independent from politics and less knowledgeable of how to deal with liquidity crises. It ended up tightening monetary policy instead of loosening it to ease the economic contraction, exacerbating employment problems for a long time. Keep in mind as well that this period of unemployment over 10% lasted for over a decade or so; it's not quite feasible to tell what all the factors were that caused the high unemployment, especially without official data on unemployment for a lot of that period.