One possible reason could be a lifestyle change. For example, if she starts using drugs or excessive alcohol, it can take a toll on her appearance. Bad diet and lack of exercise can also lead to weight gain, skin problems and a generally unkempt look.
Well, it might be due to a powerful magical artifact. She could have accidentally touched or used an enchanted item that had the power to turn humans into animals, like a cursed amulet. Or, in a more spiritual sense, it could be a connection to a spirit world where cats are guardians and she has been chosen to take on that form for a special mission. There could also be a deal made with a supernatural entity that she has to fulfill, and the transformation is part of the deal.
Another reason might be related to some kind of spiritual or mythical connection. In certain mythologies, there are beings that can transform between human and animal forms. She could be related to such a being, and due to some trigger, like reaching a certain age or entering a specific place, she turns into a cat.
In some stories, it could be a curse. Maybe the boy's family angered a powerful being, and as a punishment, the boy is turned into a girl. This then sets up a story of trying to break the curse and restore his original form, which often involves a long and difficult journey filled with challenges.
It could be due to a strong emotional experience. Maybe the boy had a close female friend who passed away, and in order to keep her memory alive or feel closer to her, he started to transform into a girl. Or perhaps he was constantly exposed to female role models who inspired him so much that he wanted to be like them. Also, in some cases, it could be a form of escapism from the pressures of being a boy, like if he was being pressured to be tough and masculine all the time.
Maybe she was under extreme stress or fear. Stress can have a physical impact on the body and disrupt normal bodily functions. For example, if she was in a very scary situation like being chased, her body might go into a fight - or - flight response and this could potentially lead to such an accident.
One possible reason could be a deviation from the original essence of the Harry Potter series. Some fanfictions might change the characters' personalities too much or create out - of - character relationships that hardcore fans dislike. Another reason could be poor writing quality. If the fanfictions are full of grammar mistakes and plot holes, it can turn readers away. Also, some fanfictions might explore dark or disturbing themes that go against the generally positive and magical feel of the original books, causing a negative reaction from the fans.
Maybe the girl wanted a partner who could understand her world better. If she was into typically female - dominated activities or interests, she might have thought that by making her boyfriend more like a girl, he would be more empathetic towards her experiences.
In some fictional stories, a curse can also be the cause. Maybe an evil witch or wizard put a curse on the man because of some past conflict or just for their own malicious purposes.
In some fictional stories, scientific experiments gone wrong might lead to such a change. Maybe a scientist was trying to develop a new gene - editing technique and something unexpected happened to the girl who was part of the experiment.
One possible reason could be that the female dragon sees something in the human girl that she wants to preserve in a dragon form. Maybe the girl has a special quality like great courage or a pure heart that the dragon thinks would be enhanced as a dragon. Another reason might be that the dragon is lonely and wants a companion of her own kind, so she turns the girl into a dragon.