Birkenstocks are a type of sandal (origin is Germany) that are popular in some quarters, especially university towns (I bought mine in Chapel Hill). I think they really became popular during the 60's. Anyway, it's not that unusual to hear someone of a conservative bent say something like, "He's one of those Birkenstock-wearing liberals from over at the University."
I guess it's another one of those Southern "things." For many years, "down here," we've noticed the tendency for folks transplanted from Yankeedom to wear sandals with dark socks. The custom here is either wear no socks at all with sandals or possibly white socks.
Yeah, and they know they're commented upon; but they keep on doing it! It's like wearing Depends underneath a speedo . . . the two just don't go together.