One possible reason could be that she sacrifices herself to save Naruto from a powerful enemy. In the heat of battle, she might put herself in harm's way to ensure Naruto's survival.
Maybe she gets caught in the crossfire of a large - scale conflict. With all the chaos and destruction, she could be in the wrong place at the wrong time, and despite her skills, she is unable to escape death. It could also be a result of a mission gone wrong where she underestimated the enemy's strength.
Naruto might become more determined. With Sakura's death, he could see the harsh reality of the ninja world more clearly. It might make him more resolved to bring peace, as he has now lost someone important. He may also become more protective of his remaining friends, vowing not to let any more harm come to them.
One possible reason could be extreme jealousy. Sakura might be so blindly in love with Sasuke that she sees Naruto as a threat to Sasuke's attention or her relationship with Sasuke. Maybe she thinks Naruto is getting in the way of Sasuke's return to the village or her attempts to win Sasuke back.
One reason could be jealousy. If in the fanfic, Sakura gets closer to someone else Naruto cares about, like Sasuke in a different way than he expected, it might make him hate her. For example, if Sakura teams up with Sasuke against Naruto's wishes in the fan - created plot.
In some fanfictions, it could be due to unrequited love. If Naruto has deep feelings for Sakura but she continuously rejects him in a cruel or thoughtless way, Naruto might feel angry enough to seek revenge. For example, she could publicly humiliate him for his feelings, which would hurt Naruto deeply and lead to thoughts of revenge. Another reason could be a misunderstanding. Suppose Sakura is misjudged by Naruto as having done something terrible when in fact she was innocent. But Naruto, in his ignorance, decides to take revenge. This can create a lot of drama and conflict in the fanfiction.
One reason could be that Naruto has always been more attracted to those who understand his outcast past better. Sakura doesn't really have that kind of connection with him as she comes from a more normal background. So there's a lack of that fundamental understanding between them which hinders any romantic feelings.
One possible reason could be a misunderstanding. Maybe she was misled by false information about Naruto being a threat to the village. Another reason might be that she was under some kind of mind control. In some fanfictions, dark magic or evil entities can take over a character's mind and make them act out of character.
Maybe she has some unresolved issues from her past, like her unrequited love for Sasuke that still lingers in the back of her mind. This could make her question her relationship with Naruto and lead to her leaving.
Well, it could be that in this fanfiction, Naruto has some sort of hidden agenda. He might call Sakura ugly as a form of reverse psychology. He could be thinking that by doing so, she'll prove him wrong and become more confident. Or perhaps there's some miscommunication or misunderstanding between them. For example, Naruto misheard someone talking about Sakura in a negative way regarding her looks and he wrongly believes it, so he says she's ugly.
Perhaps there was some sort of dark influence at play. In the world of Naruto, there are often dark forces and genjutsu (illusion techniques). It's possible that Naruto was under the influence of a genjutsu that made him see Sakura as a threat, so he punched her in self - defense without realizing it was an illusion.
Sakura's obsession with Sasuke. Naruto might be fed up with how Sakura always puts Sasuke first, ignoring the people around her who truly care about her, like Naruto himself. So he tells her off to make her realize her wrong priorities.