I think the reason that eating disorders crop up in seminary is because the girls are away from home, and there is no one watching over them to make sure they are eating.
Its obviously the girls who had this tendency before seminary, but in seminary they have the opportunity to come out. No one is sitting down at the table with you and seeing if you are eating. You could come back to the dorm at 10 at night, and no one has to know were you were or what you ate.