There are a lot of hypotheses, but they're things that are virtually impossible to quantitatively measure.
It's extremely common for girls to go through periods of body dysmorphia right before and during the early stages of puberty. It's very common for girls in the 11 to 14 range to be very unhappy with their bodies, scared of the changes occurring - both physically and socially - and to wish they were boys because it would be easier.
Speculatively, this has a lot to do with changing social expectations, and restrictions that show up for girls when they begin to develop. Where they used to be allowed to go out and play with any children, climb trees, fall down, wear whatever they want, and basically be free to be children... now they are expected to dress more demurely and their interactions with boys start to be monitored as a risk. When they start to develop breasts, girls get treated differently. Boys and adult men start to look at them differently, and start to look at them as sexual objects, frequently well before those girls develop any real sexuality at all. They have to be much more aware of their clothing and what they're doing... so that they don't accidentally expose some forbidden flesh and encourage male attention. There's a subconscious (and sometimes blatant) assumption that it's the
girl's fault if a boy or adult man focuses inappropriate attention on her. It's more or less "It's your fault, little 12 yo Emily, that the grown-up man started hovering around you and making you uncomfortable - look what you're wearing! You can clearly see that you have boobs, with that skimpy tank top! You should dress more demurely so that you don't tempt adults like that".
Boys don't experience that kind of shift in expectations, and they don't get subtly blamed for their bodies developing and affecting the sexual attention of other people and adults. Boys don't end up with their movements being proscribed and monitored because they've begun maturity. Boys tend to get more freedoms as they enter puberty, not less.
Additionally, whether it's nature or nurture, girls
cluster in a way that boys don't. Boys will have groups of friends, but girls have cliques. I'm not even going to try to speculate on why. But the effect of that clustering is "social contagion". There's a lot of "copying" and "mirroring" among girls. There's a lot of pressure and expectation to conform to ones peer group, and actions and behaviors that produce positive reinforcement tend to be quickly adopted by other girls in that cluster. So if one girl wears a new style of sweater, and gets compliments on it, other girls in her cluster are very likely to adopt the same style of sweater. This extends to behavior and presentation as well. So there's at least some historical reason to think that a girl identifying as transgender who gets positive responses to this (you're so brave and stunning, so wonderful) is likely to influence other girls to follow in her footsteps. I doubt it's even a conscious thing - most of the mirroring and conformity behaviors of pubescent girls aren't something they're cognizant of.
An additional potential complicator has to do with how sexual orientation, and sexuality in general, expresses in girls as opposed to boys. This is a spot where I have a lot less knowledge, so I'm mostly reiterating what I've read from other people, so take it with a grain of salt.
My understanding is that when sexuality begins to express in pubescent kids, they tend to show up differently. Boys sexuality is outwardly focused, desire-based. Girls sexuality tends to be more inwardly focused. It's hard to put into words, because it's not something I have done any real research on

. I think it basically comes down to boys becoming sexually aware, and going through stages of recognizing and identifying what they want and desire. Girls go more through stages of trying to identify what about them is desirable by others. Again, I have no idea whether this is nature or nurture or a combination thereof. The point here is that when kids develop sexuality, if they are homosexual, they react differently. Boys who are homosexual tend to assume that there's something wrong with
what they want, and they mask that by essentially pretending that they don't want other boys, they want girls. On the other hand, girls who are homosexual tend to assume that there's something
wrong with their bodies and it's reactions. My understanding is that young gay boys tend to be closeted in a commonly understood way, whereas young lesbians tend to go through very internally-focused anxiety and think that there's something wrong with their bodies. That could, speculatively, amplify the pre-existing tendency of girls to have periods of body dysmorphia... and in an era where being trans is lauded as a wonderful thing, and is constantly affirmed, there's a concern that a lot of young lesbians are essentially being walked through conversion therapy and instead of being allowed to come to terms with being a cisgender lesbian, they become convinced (with the assistance of therapists who fast-track and affirm) that they're actually transgender heterosexuals.