I used to have it as an effect of my avoidant disorder. I would do it while ruminating or fantasizing. I say used to, because I eventually damaged my hip joint from the constant rotational motion. Not severely, but the damage is done. That really drove home how extreme and unwell the behavior was. Which was an disturbing and sad realization.

For a long time I thought it was just thinking about stuff and stretching my legs and moving, rather than sitting all the time. I hadn't noticed how bad it had gotten over the years.

It could be depression, I suppose. Depression makes useless activities seem okay compared to seemingly hopeless or overwhelming activities that are useful. But as you've heard, anxiety would be most people's first guess.