In some fanfictions, external influences play a role. Maybe there's a character who manipulates Ron, convinces him that Harry and Hermione are not his true friends. This character could be using Ron for their own evil purposes, like getting to Harry for some dark magic reason.
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.
In some fanfictions, it might be because of a love triangle gone wrong. Suppose Ron had unrequited feelings for Hermione and thought she favored Harry too much. And Harry, perhaps misinterpreting Hermione's actions in a crucial situation, joined Ron in betraying her. This would be a complex and emotional reason for the betrayal.
In some fanfics, Ron could be influenced by dark magic or dark forces. Maybe he gets cursed, and as a result, his mind is twisted to turn against Harry. It could start with small acts of undermining Harry and escalate to a full - blown betrayal. This is an interesting concept as it shows how external forces can change a character's nature.
One common theme is trust. When Ron betrays Hermione, it often shatters the trust they've built. Another is heartbreak. Hermione usually experiences a great deal of pain. For example, if Ron cheats on her, it breaks her heart. And there's also the theme of forgiveness. Sometimes the story is about whether Hermione can forgive Ron for his betrayal.
Perhaps Ron was influenced by outside forces. There could have been dark magic or some other characters manipulating him. It's not uncommon in the wizarding world. Some powerful dark wizards might have preyed on Ron's insecurities to make him turn against Harry.
A major reason could be differences in their approaches to fighting the dark forces. Maybe in the fanfic, Hermione wants to use a more diplomatic and academic approach, Ron is hesitant and just wants to follow the status quo, and Harry wants a more direct and action - based approach. Their inability to reconcile these differences can lead to Harry being angry. For example, when they are planning how to deal with a new dark wizard threat, their arguments and lack of compromise can make Harry frustrated and angry with them.
Hermione might seek revenge. Given her intelligence and magical abilities, she could use her skills to get back at Harry and Ron for their betrayal. This could lead to a big rift in their friendship and even endanger their common goals in the wizarding world. She might also side with other characters who are against Harry and Ron, changing the whole dynamic of the story.
One common plot twist is that Ron and Hermione are actually under the Imperius Curse. So they don't have control over their actions when they betray Harry. Another could be that they are trying to protect Harry in a very wrong way. They might think that by distancing themselves from him or even giving false information about him, they can keep him safe from a greater threat.
Maybe a new dark wizard or witch emerges. This character is extremely powerful and manipulative. They target Ron and Hermione because they are Harry's closest friends. By turning them against Harry, this antagonist hopes to isolate Harry and make him easier to defeat. The new antagonist could have some unique magical abilities that allow them to control the minds of others or create very convincing false scenarios.
It could be that a prophecy reveals something that makes Ron and Hermione think Harry is a danger to the wizarding world. They might decide to betray him in order to protect the greater good. However, as time goes on, they could realize their mistake. But by then, the damage to Harry's trust would already be done, and he would have a hard time forgiving them.
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.