Curses are also common in such stories. An angry sorcerer or an evil spirit might curse a boy out of spite or revenge. The curse could be designed to change his personality and appearance in a way that makes him seem sissy, and of course, the boy would be unwilling as it's not his choice.
In a school setting, a group of mean kids play a prank on a boy. They use some sort of experimental chemical from the science lab. This chemical alters his hormones in a way that he starts to develop sissy - like traits. He is very unwilling as he is bullied more because of this change. But then he finds a true friend who stands by him and they work together to find a way to reverse the effects.
One possible reason could be the teacher's own biases. Maybe the teacher has some old - fashioned or wrong ideas about gender roles and is trying to enforce them on the boy. For example, if the teacher believes that boys should be more 'feminine' in certain aspects like behavior or interests.
In some fictional stories, it might be a wrong exploration of power dynamics. The person who forces the boy into this situation might be portrayed as a villain with a warped sense of power and control. However, this is still a very inappropriate concept to explore in this way. And it also could be related to some misaligned ideas about gender roles, which are completely wrong. We should promote positive and healthy gender concepts.
It could be due to character flaws or bad choices they make. Maybe they are lured by money or fame in a fictional setting.
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.
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.
Some kind of scientific experiment error. For example, if there was research on hormonal or genetic manipulation and something went wrong during the process.
Magic gone wrong is a common reason. Just like a magician who miscalculates a spell and accidentally turns someone into a baby boy. Another reason could be a curse. For example, if someone angers an evil witch, she might put a curse on that person to turn them into a baby boy as a form of punishment.
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.
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.