I saw a post on Mumsnet a few days ago from a mother who remarked that their little boy had been quite into "girly" toys and colours etc and they'd been cool with that and just let him do his thing. But as soon as he started going to school she noticed that he stopped doing that and wanted "boy" stuff - camouflage patterns instead of pink etc. It's not the only time I've heard that sort of thing.
So I'd say never underestimate peer pressure, especially at times like a new school or going through puberty, when they're vulnerable because they haven't found their tribe and don't know where or even whether they'll fit in.
Especially in adolescence, they're likely to be pushing against their parents; even in a healthy family, that's a normal part of growing up. So is feeling that you are neither fish, flesh nor good red herring and that Mum and Dad will never understand.
Most teenage rebellions end harmlessly. Some of the ones that don't are ones where bad actors or even well meaning fools drive a wedge between the child and their parents. The men who convince an underage girl that illicit, unprotected sex is liberation. The drug dealers. The transactivists who say hormones and surgery will make you happy and anyone who says not is a hateful enemy and should be cut out of your life.
So don't assume that wanting to transition is always down to bad parenting. That's too simplistic.