It seems a perfect (manufactured) storm: the need to regain control, the desire for thin body shapes, concerns over animal welfare and the environment, the desire to reflect the sacredness of life and the programming of animal fats as unhealthy. I think it would take more than a cabal of average thinkers to come up with such a successful plan.
I am interested in those who came to vegetarianism, apparently for no other reason than a dislike of meat, usually the flavour or texture. My teen-aged niece, for example, doesn't seem too concerned about changing her body image (if anything, I worry she thinks too highly of her looks), she has no spiritual leanings and lived a sheltered life, so she isn't all that aware of environmental or animal welfare concerns.
However, a couple of years ago, at 14, she decided to adopt a vegetarian-ish diet. She just didn't like any meat. When she was young, she was quite picky, but it seemed that her tastes kept changing, so you'd never know what her preferences were from one month to the next. The only thing I can link is that she started having problems with her stepfather, who, in our estimation, was emotionally abusive and her mother seemed to side with her partner. Since the stepfather often cooked the meats, she may have made a subconscious connection between him and meat.
She's currently living with us and we are always exposing her to new recipes with meat, in the hopes she will find something she likes. She appears to give an honest effort at trying but just doesn't like the taste, no matter what we prepare. Since she isn't avoiding meat out of moral or ethical reasons, we don't feel wrong in sneaking in bone broth or animal fats into "vegetarian" sauces we prepare for her. We've even told her we do it from time to time, and she wasn't upset.
So, if her taste is somehow correlated to an abusive parent and the food he prepared, we might have another contributing factor for vegetarianism. In many cultures, men play a role in preparing meats, be it cooking it or simply carving it once it's been cooked. Look at barbecuing - most is performed by men. So, in a patriarchal society where men cook meats, vegetarianism might be a natural result for those coping with dysfunctional fathers.
As a side thought, I'm curious to know what it took to get an entire people, Hindus for example, to adopt a vegetarian diet. Could it be as simple as a prophet dictating the new rules or does it require a slow transition, building upon multiple strategies?
Gonzo