It could be a curse. Maybe his family had some old enemies who put a curse on him. The curse was designed to change his gender as a form of punishment or revenge, and as a result, he turned into a cute girl.
A scientific experiment gone wrong. If he was in a science - fiction type of setting, perhaps some crazy scientist was testing a new device or formula that had unexpected side effects of gender transformation.
Magic could be one reason. Like in a fairy - tale world, a magic spell might be cast on the boy. Another reason could be a scientific experiment gone wrong. Maybe some sort of genetic - altering experiment that was supposed to do something else but instead turned the boy into a girl.
Some kind of scientific experiment error. For example, if there was research on hormonal or genetic manipulation and something went wrong during the process.
One possible reason could be a magic spell. Maybe a wizard or a witch cast a spell on the boy for some reason, like revenge or as part of a test.
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 that the parents have some unfulfilled desires related to having a girl. Maybe they always dreamed of having a daughter and are trying to create that situation by changing their son's appearance or upbringing. But this is a wrong approach as it doesn't respect the child's identity.
Magic potions. In fairy - tales, magic potions often cause strange transformations. A boy might accidentally drink a potion meant for something else and turn into a girl.
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.
One possible reason could be a magical accident. Maybe mom was trying to do something else with a spell or charm and it backfired, turning the person into a girl.
One possible reason could be for medical reasons. For example, if the boy has a genetic disorder where he has ambiguous genitalia and the medical decision is to raise the child as a girl for better physical and psychological well - being in the long run. Another reason might be in the context of a fictional story, to explore themes of identity, acceptance, and transformation.
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.