racerX said..
I would hazard to guess its the same mechanism that causes other waves to form, I wonder even on bigger beaches if 'sets' begin to form in the sand...

when landsailing on really wide beaches or really long wide beaches where the tide has gone out for a long time you start to get low flat dunes say 100mm high forming that move along the beach more often at about 45 degrees rather than straight down wind. if there is a racing turning mark tyou can judge the speed they move along. if it stupid windy,eg Rada Tilly 2008 world champs, the beach just flattens out and you sail through a 300mm high sandy wind horizon that you can physically see like a smoke trail in a wind tunnel