It's not like bulimia nervosa is voluntary in the first place; it is a bit like smoking, after awhile you "have" to smoke and find it hard to stop. That one's tummy would physically get in the "habit" too wouldn't surprise me. One's symptoms are like that. Just like we have weird dreams, the unconscious and semi-counscious parts of our minds are paying more attention than we know and more powerful than we usually imagine. Teach the mind a new "trick" and it will happily oblige by taking it over and it's your new habit, whether you want it or not.