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.
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 a misunderstanding. Maybe Inuyasha misinterprets Kagome's actions or words. For example, if Kagome is trying to help a new character who seems to have a connection to the Shikon Jewel, Inuyasha might think she's being disloyal and thus betray her. Another reason could be some sort of dark influence. There could be an evil force that makes Inuyasha act out of character and turn against Kagome.
Jealousy is a big one. Ron often feels like he's in Harry and Hermione's shadow. He might see their achievements and feel left out or unimportant, which could lead to his betrayal.
She might become more distrustful. After being betrayed, it's natural for her to have a hard time trusting others again.
One reason could be power. Maybe he was offered a great deal of power by an outside force in exchange for betraying the harem. Another could be jealousy. If there was someone in the harem who was getting more attention or was more successful than him in some way, he might have been driven by envy to betray them.
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.
Hermione could react by using her intelligence to expose Fleur's betrayal. She might start gathering evidence of what Fleur has done wrong and present it to their mutual friends or to an authority figure in the wizarding world.
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.
One possible reason could be jealousy. Maybe some of his former allies were envious of his fame and hero status. Another reason might be political manipulation. There could be those who wanted to use his downfall for their own gain in the post - war wizarding world.