A family obligation could play a part. Maybe their families are threatened by a certain group, and the only way to protect them is to side against Harry. In the world of fanfiction, there could be all sorts of complex family dynamics at play. It could be that they are tricked into believing that Harry is somehow involved in the threat against their families, leading to their betrayal.
Power could be a reason. If there's a chance for Ron and Hermione to gain a great deal of power by turning against Harry, some fanfictions might explore this. For instance, a new dark power might offer them something they've always desired, like the ability to reform the wizarding world in their own image. They might think Harry would oppose this, so they betray him to get what they want.
It could be due to a misunderstanding. Suppose Ron overhears a conversation between Harry and Hermione that he misinterprets completely. He might think they are plotting against him or don't really consider him a true friend. So in a fit of anger and hurt, he decides to betray them.
Jealousy might be a motive. If Hermione is constantly outshining them in academics or getting more praise from teachers, they could become envious and turn against her.
In some fanfiction scenarios, it could be a matter of different ideals. Suppose there is a new movement in the wizarding world that Ron and Hermione believe in, but Harry opposes it. Their loyalty to this new ideology could be so strong that they see Harry as an obstacle and decide to betray him. It could also be that they are under the influence of a powerful and evil spell that warps their minds and makes them act against Harry without their full awareness.
It's possible that Harry hates it because he has a certain image of Ron and Hermione in his mind. Fanfiction can sometimes create versions of them that are completely different from what he knows. For example, some fanfiction might focus on a love triangle that he doesn't think exists in their real relationship. This divergence from his perception could lead to his dislike.
One possible reason could be a misinterpretation of their relationships. Maybe the author wants to explore a different dynamic where Harry and Hermione bond over Ron's perceived flaws, like his occasional laziness or his insecurities that might come across as annoying to them in this fictional scenario.
Some people might just want to explore alternative scenarios for the sake of creativity. They may think it's interesting to see how the story would unfold if there was animosity between them.
It could be that Harry has been influenced by some external force. In fanfiction, there could be a new dark wizard or a magical object that is affecting his mind. This new influence might make him think that Ron and Hermione are in danger if he stays close to them, so he ignores them to keep them safe.
Another possibility is that the fanfiction writer wants to explore a different side of their relationships. By having Hermione and Ron hurt Harry, it shows the complexity of human (or in this case, wizard) emotions. It could be a way to show that even the strongest friendships can have cracks and that mistakes can be made. It also allows for character development as Harry has to deal with this new situation, and Hermione and Ron may later realize their mistakes and try to make amends.
One reason could be that Hermione is a strong - willed character. In fanfiction, Ron might be depicted in a way that undermines her independence. She wants to be seen as her own person, not just as Ron's love interest.
One reason could be a divergence from the original character traits. In the original story, Hermione and Ron have a complex relationship with love and friendship. Fanfiction that misrepresents Hermione's feelings towards Ron might go against the essence of her character. Maybe the fanfiction portrays her as hating Ron in a way that doesn't fit with her growth and development in the official series.