I have been a folkie (in the sense of singing songs sung by people known to me) since 1939.
I have had a beard, without interruption, since 1965.
I wear sandals in a narrow range of circumstances, but my usual footwear is slippers indoors & boots outdoors.

There was another thread about this a while ago. The most plausible suggestion on it, IMO, was that the common cause is independence of mind. People who are willing to be musical deviants are likely to be willing to be sartorial & grooming deviants.