I always thought it was because they were scratching their backs. My cat Shadow (who is currently on my lap stealing all my heat) loves to get into the basement where there is bare concrete, and roll over, scratching his back on the concrete.
My own opinion I suppose. Others may have differing ideas?
Cute cat! Thanks for sharing, enjoy the weekend.
That makes a lot of sense and I notice that sometimes too, but he just prefers to sleep this way.