TaxProfBlog has a great post with several discussion on why professors tend to be so dang liberal.
The political liberalism of professors—an important occupational group and anomaly according to traditional theories of class politics—has long puzzled sociologists. To shed new light on the subject, we review research on professorial politics over the past half-century, identifying the main hypotheses that have been proposed to account for professorial liberalism. Using regression decomposition, we examine hypothesized predictors of the political gap between professors and other Americans using General Social Survey data pooled from 1974-2008. Results indicate that professors are more liberal than other Americans because a higher proportion possess advanced educational credentials, exhibit a disparity between their levels of education and income, identify as Jewish, non-religious, or non-theologically conservative Protestant, and express greater tolerance for controversial ideas. Together, the variables linked to our hypotheses account for about 43% of the political gap between professors and other Americans. We conclude by outlining a new theory of professorial politics that integrates these findings, moves beyond existing approaches, and sets an agenda for future research.
See also Professor Is a Label That Leans to the Left, New View of Faculty Liberalism, The Academic Destiny of Educated, Irreligious, Jewish, Tolerant Non-Capitalists, and Lefty Loosey, Righty Tighty: Sociologists Try to Explain the Political Orientation of the Academy.