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.
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.
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.
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.
In fanfiction scenarios where Hermione is betrayed by the Order, it could be related to a prophecy. Maybe there is a prophecy that involves Hermione in a way that the Order misinterprets. They think that by betraying her, they are saving the wizarding world. Or perhaps there is a conflict of ideology. Hermione might have some new ideas or approaches to magic that the more traditional members of the Order oppose. And they see her as a danger to their established ways. Also, personal grudges among the Order members could spill over. If someone in the Order has a long - standing grudge against Hermione for some past incident, they could convince others to betray her.
One possible reason could be jealousy. Maybe someone in Ash's group is jealous of his skills or popularity. For example, if there's a character who always feels overshadowed by Ash, they might betray him out of envy. Another reason could be a misunderstanding. Perhaps Ash was misinterpreted while making a crucial decision, and that led to the betrayal.
One possible reason could be jealousy among other characters. Maybe there are those who envy Percy's powers and leadership, Jason's status as a hero, or Nico's unique abilities. Another reason might be a misunderstanding. For example, they could be wrongly accused of a crime they didn't commit, which leads to others turning against them.
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.