It could be related to the prophecy or the overall balance of power in the ninja world. She might think that by siding with Sasuke, she can influence the outcome in a way that benefits the world more. For example, if there was a prophecy about Sasuke being key to some great change, Fem Kyuubi might believe that betraying Naruto to support Sasuke is for the greater good.
One possible reason could be jealousy. Maybe her friends are envious of Hermione's intelligence and magical abilities. In a fanfiction, this could lead them to betray her to gain an advantage or to make themselves look better. Another reason might be a misunderstanding. A miscommunication or false assumption could cause her friends to turn against her.
One possible reason could be that it goes against the original dynamic between Naruto and Sasuke. In the original story, their relationship is based on a deep male - male bond of rivalry and friendship. Changing Sasuke to a female might disrupt that established connection.
Maybe they were forced into it by some dark magic or a powerful dark wizard. They could have been blackmailed into betraying Harry to save their own lives or the lives of others they cared about.
It could also be due to competition. Sabrina might see Ash as a threat to her status as a powerful trainer. If Ash starts to gain more recognition and success, Sabrina, being prideful, could feel betrayed as if Ash is trying to overshadow her and thus take actions against him.
Well, in fanfiction, the author often likes to create complex and unexpected plot twists. It could be that Naruto's loyalty to his own values and beliefs clashes with the goals and ethics of Rias, Sona, and Ravel. For instance, Naruto may be against certain practices that they engage in, like how they handle power or deal with other groups. This ideological difference could lead to their betrayal. Also, there could be a love triangle or some sort of relationship drama involved. If Naruto is involved with someone they all like or if there are misunderstandings in relationships, it could spark the betrayal.
Well, it could be that Naruto's friends are influenced by a dark power. This power could twist their perception of Naruto, making them see him as a threat instead of a friend. Or perhaps there's a prophecy that Naruto is supposed to be the cause of some great disaster, and his friends, in an attempt to prevent it, decide to betray him. It's also possible that a new character enters the story and sows seeds of discord among them, leading to the betrayal.
It could be due to the fact that Naruto is always in the spotlight. Fem Sasuke might be obsessed because she wants to be like him in some ways or wants to understand what makes him so special. Also, their rivalry in the original series might translate into a different kind of intense feeling in this fanfiction, like an obsession.
Sometimes, it could be a matter of different loyalties. Naruto's friends might have family or other obligations that conflict with their loyalty to him. For instance, if their families are threatened by someone who demands they betray Naruto, they might feel compelled to do so out of a sense of protecting their loved ones.
One possible reason could be jealousy. Maybe Percy has some special abilities or achievements that make others envious. For example, his connection to the sea could be seen as a huge advantage that others might resent. Another reason could be misinformation. Someone might spread false rumors about Percy, leading others to believe he's done something wrong when he hasn't. It could also be that there's a misunderstanding about his relationship with Annabeth. Some might think he's getting special treatment from her or the other way around, which causes them to turn against him.
Perhaps there is some sort of political intrigue within the 'light side'. There could be factions that see Hermione as a threat to their own power and influence. So, in order to maintain their position, they choose to betray her.