College classes don't make people liberal, the rather libertine social environment does.
Realisticidealist has found the main point of why college makes people more liberal. It's mostly not because some professor rams socialism down your throat (as some occasionally do, but I doubt that changes too many people's minds anyway). It's because of the campus environment where binge drinking is nearly ubiquitous and condoms are handed out that say things like "Love is Never Wrong", and the university residential housing staff is blatantly cheerleading liberal social causes. That's why college makes people liberals, folks. It's much more from that than it is from some random sociology professor.
