One possible reason could be bullying. If she was constantly harassed at school or in her social circle, it might have driven her to such a desperate act. Another reason might be family issues, like parental neglect or abuse.
Well, it could be due to a sense of isolation. Harry has always been somewhat of an outcast, even within the wizarding community. He might feel that he can't truly connect with others on a deep level. Additionally, in some fanfic interpretations, the author might be showing how the prophecy and his role as 'the Chosen One' has burdened him to the point of despair. It's like he has no control over his own life and destiny, which could lead to suicidal thoughts.
One possible reason could be for ransom. Some kidnappers target girls from wealthy families, thinking they can get a large sum of money in exchange for the girl's safe return. For example, if a girl's parents are business tycoons, the kidnapper might believe they would pay millions to get their daughter back.
One reason could be a misinterpretation of the characters' struggles. In the original series, the characters face many difficulties, but they always find a way to persevere. However, some fanfiction writers may take those struggles too far and end up with the idea of suicide. Also, there might be a lack of understanding of the impact such content can have on the readers, especially the younger ones.
One possible reason could be a misunderstanding or misinformation at birth. For example, if there were ambiguous genitalia at birth, the doctors or parents might have made a wrong assumption about the baby's gender. Another reason could be some kind of psychological experiment by the parents, though this is very unethical. They might have wanted to prove a point about gender being a social construct.
Sometimes, it could be related to family values. For example, if a family highly values modesty and the girl dresses or behaves in a way that goes against that, a haircut can be a form of punishment. It's a physical change that can also potentially lead to a change in her attitude and behavior, making her more in line with the family's values.
One possible reason could be severe depression. Prison life can be very tough, and if a prisoner is constantly feeling down, hopeless, and isolated, it might lead to suicidal thoughts. Another reason could be facing a long prison sentence. Knowing they have to spend a large part of their life in prison can be overwhelming.
One reason could be revenge. Someone might want to take revenge on the boy or his family by changing his gender. Another reason could be a crazy experiment. Some mad scientist - like character in a story might think it's an interesting experiment to see how a boy would adapt to being a girl.
One possible reason could be jealousy. Maybe there was a boy both girls liked in the true story, and this led to the fight. Another reason might be a misunderstanding. For example, one girl thought the other girl spread rumors about her which were actually false. It could also be due to competition, like in a school setting for grades or popularity.
Often, a lack of proper communication skills can be the root cause. Girls might not know how to express their feelings or resolve conflicts in a healthy way. So, when there is a minor issue like sharing a toy or a difference in opinion, instead of talking it out, they end up in a catfight. And in the midst of all the chaos, a wedgie can happen as a form of aggression during the fight.
Maybe she faced a huge heartbreak, like losing someone she deeply loved.