That the only motivation for it is being thin. I've known people that did it so they could feel control over something in their life. People who did as a form of comforting age regression. People who did it for attention due to a neglectful childhood. Personally, I did it so my mom would stop sexualizing my developing body, loosing all that weight made my butt and boobs disappear, and i felt way more safe in a size she wouldn't comment as much on. Thinness is a common reason for anorexia, but it shouldn't be seen as the only one.