Thanks everyone!

My friend's cat lives with three other cats, none of which do this kind of thing...on the other hand, the other three are males. I wonder if it's a "girl thing" that has something to do with mating? Perhaps being stimulated on the lower back has something males do when "courting" out in the wild.

Of course, all the cats have been spayed or neutered, but the behavior may still be a holdover before cats discovered their ideal servants (us).