I don't have any theories. It does look like they are rubbing/scraping in pheromones maybe to ward off would be robbers or warn they are a strong colony. But it's never been proven. It is always strong healthy colonies that washboard. I make a note of it so that I can raise queens from those hives the following year. Now all my hives washboard. One theory I like is the behavior is kind of like your dog walking into the living room and circling, patting down imaginary grass to lay down. They don't need to do that, but they still do as that behavior was passed down from ancestors a million years ago. It could be that it's a passed down behavior from millions of years ago that makes them clean up from something that affected them a very long time ago. Maybe a long extinct plant or tree.
