Actually, I recall from my stint as a teenager that sitting oddly tended to make my back feel better. Growing pains and such. There’s also that awkward phase where the adult-sized chairs are not quite comfortable for the not-adult-sized teenager.
I find myself doing that once in a while at work. I think it’s mainly a holdover from my youth when I used to slouch a lot, for which my mother was always giving me hell. I also find that whenever I do it I tend to get an impulse to have a nap.