Indeed, the primary reason that the 1960s were 'more liberal' than the 1950s is simply that the GI Generation was coming into power and displacing the 'Lost Generation' of Truman and Eisenhower, which was always fairly reactive collectively, if you put much stock in such things.
Voting power wise the GI generation was already dominant by the late 40s, and Truman was a pretty liberal president as well . The 1950s were conservative (except on economic issues ) because of Red Scare , other then that the 1950s weren't that conservative .
In fact the 1950s were more liberal then the 1980s ,2000s and in many ways the 1990s as well.