I don't think the veganism-factor should automatically be assumed to be connected with wanting control over one's life (not that you've "automatically" done that) beyond - maybe - just wanting control over one's own diet.
Not only are there a lot of people (and these days, more and more, I think) who struggle with the idea of eating other creatures. Also, young people are often either aiming to be "the latest form of enlightened", so even it it's only because of health reasons, many will tend to follow trends.
Although I know there are boys or young men who don't care a lot about food in the first place; I think there's a thing with girls and women of all ages when they have a small frame, don't have a particularly big appetite, and aren't the kind of people who have a big interest in food-as-entertainment and/or food-for-the-sake-of-more-than just getting rid of hunger and having other interests in life.
I'm not young, and I'm far from under-120 lbs I was when I was young; but between not having a big appetite or big love of/interest in food in the first place; and any other factors there are with food I find too heavy, too hard-to-digest, or too whatever-else for my preferences (which are pretty much the same as they were when I was six, and pretty much similar to the preferences of a lot of six-year-olds); I can see how easy it would be to lose whatever appetite, say, a young woman has to the point where it's hard to even imagine eating or even drinking much of anything.
Just this week I've been through that kind of thing because of stress and "frazzlement", and I'm having a really hard time making sure I at least eat/drink something. Some types of stress make a person need higher calorie/fat foods. Some (enough) can make a person feel like she has absolutely no appetite at all - almost as if she has no digestive system/needs at all. People who can eat when they're not hungry often just don't understand that.
While I can see how easily a young, small, person at the mercy of adults who don't understand one thing or another could be at risk of developing an eating disorder, I tend to think (as evidenced by today's overweight kids) that stress and demands tend to make people eat more calories/fat, not less.