As someone who is doing that at the moment, the reason for me is because I treat the floor as dirty (due to cats) and want to avoid touching it with bare feet or socks. When I'm in the apartment, I wear slippers. My socks are stored inside my boots (merino wool, so doesn't need cleaning often). When I leave, my process is: remove foot from slipper, put on the sock while standing on one leg, put sock into shoe or boot, repeat for other foot.

I'd be surprised if my process is common though.