It's not suprising to me that American professors are more liberal than American population as a whole. "44 percent of professors are liberal, 46 percent moderate or centrist, and only 9 percent conservative. The corresponding figures for the American population as a whole, according to public opinion polls, are 18 percent, 49 percent, and 33 percent, suggesting that professors are on average more than twice as liberal, and only half as conservative, as the average American".
But why certain fields are more conservative than others are really puzzling (see table below, source: Neal Gross and Solon Simmons, “The Social and Political Views of American Professors”). I am surprised to find that economic professors are not the most "conservative" ones, instead Accounting and Engineering professors are
For details, read Posner and Becker's blog on this.