Definitely homophobia, and also misogyny.
I think we also have to look at the agenda of late transitioners. Some of them now wish they had transitioned earlier in life in order to pass better. Some of them badly want to get scientific proof of a "born this way" gender identity (ie female brain in male body) so that acceptance can be driven forward in the same way as has been done for gay rights. Those voices are very powerful and are getting a lot of space and time in the media.
(as an aside, I also don't find the "born this way" narrative of homosexuality either persuasive or necessary. It's been a good way of expanding rights and acceptance of gay people, but for me it's not necessary that being gay is inborn or inevitable for me to feel positively about it. Many of my gay friends are also up in arms about the same methodology being used for the trans cause because it now threatens to drive them backwards in the sense of gay men having "female" brains and gay women having "male" brains, which is pretty regressive. Anyhoo, I digress).
So nervous, worried parents with GNC children are being told that they are trans. It's attractive because it's easy - oh, right, my boy has a girl's brain. I get it. It's attractive to liberal, concerned types of parents because it's an opportunity to virtue signal what a great parent you are - look, I'm so wonderfully accepting of my child, aren't I a great parent? (sorry, that does come off harsh, but I do see this). It's attractive to conservative parents because they'd really rather not have a gay child, thanks very much. Some are being terrified into it by being told that their trans child WILL KILL THEMSELVES if everyone around them does not agree that their gender non conformity means that they were born in the wrong body.
And I think there can be darker motives. I look at the mothers who insist that their 18 month olds are trans because they grabbed a pink blanket and I think that being gender non conforming is frankly the least of that child's worries. Gender disappointment? Munchhausens? I hasten to add that I think this is rare, and the vast, VAST majority of parents are only trying to do their best by their children.
So much to unpack here, isn't there?