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.
Well, it could be for power. In the world of fanfiction, power is often a big motivator. There might be a villain who tricks someone close to Ash into betraying him in order to gain access to Ash's special abilities or his Pokémon. Or it could be related to a hidden agenda. Maybe there's a character who has a secret mission that conflicts with Ash's goals, and so they betray him to further their own plan.
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.
Ash's training could make him more determined. He has a new purpose now, which is to show those who betrayed him that they made a mistake. This determination could be seen in his more aggressive training regime and his eagerness to take on tougher opponents. He might also become more observant. Through intense training, he learns to pick up on small details in battles and in the behavior of others, which helps him avoid future betrayals.
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.
It could be that in this fanfiction, Ash had some personal issues come up. For example, he might have received some bad news from his family back home that made him lose focus on training and eventually give it up. Another reason could be that he found a new passion or interest that diverted his attention away from training.
One possible reason could be that he was forced to betray under some kind of mind control. Maybe an enemy like Hydra got hold of him and made him do things against his will. Another reason might be that he was trying to protect something or someone he loved in a very extreme way that seemed like a betrayal to others. For example, he might have made a deal with a villain to keep Pepper safe.
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.
One possible reason could be jealousy among his peers. Maybe someone was envious of Percy's skills or popularity and spread false rumors about him, leading to his betrayal. For example, if Percy had a special power or was favored by a certain group, others might feel left out and decide to turn against him.