I second this theory.
Earl was a popular name in the 1920s and 1930s, and being a one syllable name I imagine it would have made a popular middle name.
A brief search of Wikipedia using first names that were also popular at the time shows several instances of man named "
Robert Earl", "
John Earl", "
William Earl", "
Richard Earl" and "
George Earl". This seems to suggest that
Earl was once a fairly common middle name. So it's no surprise that "
James Earl" is a common combination.