It could be a curse. There are fanfictions where ancient curses are unleashed and Sasuke becomes the victim, resulting in his transformation into a girl. This curse might be related to some long - forgotten feud or dark magic in the ninja world.
One reason could be a jutsu gone wrong. Maybe an enemy uses a forbidden jutsu on Sasuke that has an unexpected side effect of changing his gender.
In some fanfictions, it could be due to a magic spell gone wrong. Maybe a wizard or a witch was trying to do something else and accidentally turned Hiccup into a dragon. Another reason could be a cursed object. Hiccup might touch or use an object that is cursed and that causes the transformation.
In some turned into a girl stories, it could be due to a curse. For example, if someone offended an old witch or a powerful spirit, they might be cursed to change into a girl. Also, there could be a strange natural phenomenon. Like in a place full of strange energy, a person could be transformed into a girl just by being there for a while.
Maybe Naruto was trying to protect Sasuke from a certain danger. There could be a curse or a powerful enemy that targets male shinobi. So, Naruto uses a risky jutsu to transform Sasuke into a girl to keep him safe. Or it could be that Naruto found a strange ancient scroll with a transformation technique that he thought was something else and when he used it on Sasuke, it turned him into a girl.
There's a fanfic called 'Girl - Sasuke'. It delves deep into how Sasuke copes with the physical and mental changes of becoming a girl. It shows his struggle to maintain his strength and status among his peers while also adapting to the new social expectations that come with being female.
One possible reason could be that he was framed for a serious offense he didn't commit. Maybe someone planted false evidence of him attacking a fellow student. Another reason might be that he continuously defied the school's strict rules in order to pursue his own goals of getting stronger faster, which the school authorities couldn't tolerate.
Magic is a common reason. Maybe a mischievous fairy or a clumsy wizard cast a spell wrongly. Another reason could be a strange scientific experiment gone wrong, like some sort of radiation or chemical reaction that alters the body's form. Or it could be a curse from an ancient artifact. If someone disturbs an old tomb and takes an enchanted item, it might put a curse on them and turn them into a little girl.
One possible reason could be that Sasuke's past traumas and his continuous search for redemption make him feel unworthy of Sakura's love. He might think he's dragging her down and decide to end the marriage for her own good.
One possible plotline could be Keith's struggle to adapt to his new female body. He might face challenges in how others perceive him now, especially among his friends and allies. For example, some might treat him differently, either with more protectiveness or with new - found romantic interest. Another plotline could involve Keith exploring the differences in how society treats women compared to men in the context of this fictional world. He could use his new perspective to fight for gender equality within the story's setting.
A common reason is battles. Sasuke is often involved in intense combat situations in fanfiction, whether it's against old foes with new powers or entirely new enemies. His determination to prove himself or protect something often leads him to take risks that result in getting hurt.
Family problems can also play a part. For example, if there's abuse or neglect at home, it can make her lose faith in goodness and turn bad. She may act out in rebellion as a way to cope with the pain.