It's both. A social norm sustains itself because if most of the people I want to be like are doing it (correlation), then I will do it (causation) too, unless I've put a lot of extra thought in and made a conscious decision to do something else.

The social animal's default mode is to follow the herd. "The herd" could have even started out as the Texas Sharpshooter's Fallacy (random correlation), but now people are motivated to join in. To assemble. To copy.