One possible reason could be self - defense. If the boy was harassing or threatening her in some way, she might react instinctively to protect herself. For example, if he was making inappropriate advances, kicking him in the balls would be a way to stop him immediately.
It could also be a result of an accident. Maybe they were playing a rough sport or game together, like soccer, and during the chaos of the game, she accidentally kicked him in that sensitive area.
Perhaps in the context of the story, there is some sort of mental or emotional instability involved. The woman could be suffering from some psychological issues that cause her to act out violently without proper control. This would then be a story about the boy's recovery and the woman's journey towards getting the help she needs.
In a neighborhood game, a boy made a really unfair move against a girl. The girl got angry and in a split - second decision, she kicked him in the balls. The boy fell to the ground, tears in his eyes. The girl was then scolded by some of the other kids for such a violent act. However, the girl argued that the boy had been wrong first and she just wanted to teach him a lesson not to be so unfair.
Maybe he was involved in some rough sports. For example, in a game like rugby where there is a lot of physical contact, a wrong move could lead to such an accident.
It can be quite shocking and maybe uncomfortable to see. It might catch readers by surprise and leave a strong impression.
The girl could get into trouble. If it happens in a school or a public place with rules, she might be disciplined. For example, she could get detention or be suspended. Also, it could damage her reputation among her peers as some might see her as too aggressive, even if the boy had provoked her in some way.
In a more metaphorical sense, it might represent a character's sudden realization of having more masculine qualities or power. For example, she could be waking up to a newfound assertiveness or a sense of being in a male - dominated world where she has to adapt to male - like characteristics to survive or succeed.
Often, it might involve elements of humor, surprise, or maybe even a bit of revenge. But it really depends on the specific manga.
It might be popular because it adds an element of shock or unexpectedness. Sometimes, such extreme actions can draw attention and create a memorable moment.
One reason could be magic. Just like in fairy tales, a magical spell or object might cause the transformation. For example, a magic potion that was drunk by mistake.