Well, it could be that in these fanfictions, a dark influence overcomes Hermione and Ron. They might be manipulated by a powerful dark wizard who makes them believe that betraying Harry is the right thing to do. It could also be a miscommunication situation where they think they are helping but end up betraying Harry. For instance, they might be tricked into believing that Harry has turned bad and needs to be stopped.
Some fanfictions play with the idea of character flaws. Hermione's intelligence and Ron's insecurities could be twisted in such a way that they betray Harry. Hermione might think her intellectual superiority gives her the right to make decisions for Harry, even if it means going against him. And Ron, with his envy sometimes, could be lured into actions that seem like betrayal. This creates a really interesting and often heart - breaking twist to the original story.
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.
Well, Harry might become very bitter. He could turn to other allies like Neville or Luna for support. He might also start to question all of his relationships and become more withdrawn.
Well, in certain fanfiction scenarios, Ron's jealousy could be a factor. He might feel overshadowed by Harry for so long that it finally drives a wedge between them, and Hermione, being in a relationship with Ron, gets caught up in it. Another possibility is that a prophecy or some ancient magic forces them to make choices that seem like a betrayal to Harry, but they believe they are doing it for the greater good.
In some fanfictions, Hermione might betray Harry due to a misunderstanding. For example, if there were false rumors spread about Harry being involved in dark magic or having a secret, evil plan. Hermione, being a stickler for the rules and the greater good in the original series, might be misled into believing these rumors and thus betray Harry.
At first, Harry might be confused. He would try to figure out what went wrong. He might search his memory for any signs that could have led to this betrayal. Then, he would likely experience a range of emotions from sadness to rage. Depending on the nature of the betrayal, he could either try to win them back by showing them the truth or decide that they are no longer worthy of his friendship and focus on his own path, maybe even leaving the wizarding world for a while to clear his head.
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.
If Ron and Hermione leave Harry in the fanfiction, it would change the entire fabric of the story. Harry would have to reevaluate his relationships with other characters. For example, he might turn to Neville for support more often. Also, the power dynamic within the wizarding world could shift. Since Ron and Hermione were important figures in the fight against Voldemort, their absence could give the Death Eaters an advantage, or it could inspire others to step up in unexpected ways. Harry would be forced to grow in ways he hadn't before, and it would be a journey full of self - discovery and new challenges.
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.
In a lot of such fanfictions, Ron could be influenced by dark magic or some outside forces. For example, a powerful dark wizard might use a spell on him to make him turn against his friends. He could then give away their secrets or plans to the enemy. This would put Harry and Hermione in great danger and they'd have to find a way to overcome the situation without Ron's help at first.