It could also be a way to show his emotional state. He might be so deeply affected by a situation, like the loss of a dear friend or a major setback in his fight against the dark forces, that he is unable to speak. His silence then becomes a powerful way to convey his inner turmoil without using words.
Perhaps it's a plot device to shift the focus. If Harry Potter stops talking, the other characters around him will have to step up and communicate more. This can change the dynamics of the story, allowing other characters to develop further. For example, Hermione might take on more of a leadership role in communication when Harry is silent.
One possible reason could be that the author wants to create a sense of mystery. By making Harry Potter stop talking, it makes the readers wonder what he is thinking or feeling, and adds an air of enigma to his character. Maybe he has witnessed something so shocking that he is rendered speechless for a while.
One possible reason could be a misunderstanding. Maybe Hermione does something that Harry misinterprets, like siding with someone he doesn't trust in a Hogwarts issue. Another reason might be jealousy. If Hermione starts getting closer to another character, Harry could feel left out and stop talking to her. Or perhaps a dark influence affects Harry's mind, making him think Hermione is a threat to him in some way.
One possible reason could be that he has been through so much in his original adventures that he's just tired. He might feel like he's constantly being put in new, often contrived, situations in the fanfiction and he simply wants to step away from all the chaos and drama.
Perhaps the writer of the fanfiction is trying to create a different character arc for Harry. Maybe he has faced so many hardships that he becomes disillusioned. For example, if he has lost too many loved ones and feels that his efforts are in vain, he might stop trying. It's a way to add a new layer of depth to his character.
One possible reason could be a misunderstanding. Maybe Sakura said something that Naruto misinterpreted as being hurtful or dismissive of his feelings. For example, if she made a comment about his abilities in a negative way during a crucial moment, it could lead to him distancing himself.
The 'Harry Potter' series is far from being a 'criminal fanfiction'. J.K. Rowling created a magical universe filled with positive values. Harry and his friends are constantly fighting against the real criminals in the story, like Voldemort and his Death Eaters. It's a story about good triumphing over evil, not about criminality.
One reason could be that he discovers a new threat and decides he can't stay with the Dursleys any longer for their safety. Another might be that he simply can't bear their mistreatment anymore and finds an opportunity to escape.
Perhaps Harry has grown tired of always following Dumbledore's instructions without fully understanding the reasons behind them. He might want to make his own decisions and take control of his own destiny, rather than relying on Dumbledore all the time.
One possible reason could be a powerful spell backfiring on him. In fanfiction, authors often play around with magic going wrong. Another reason might be extreme emotional distress. For example, if he discovers something truly shocking about his past or his friends.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.