One possible reason could be jealousy. Maybe Ron is jealous of Harry's fame and the attention he gets. Another reason might be a misunderstanding. For example, if Harry makes a decision without consulting Ron in a crucial situation, Ron could feel left out and get angry, leading to a fight.
One reason could be jealousy. Maybe Ron feels that Harry is getting too close to Hermione and he's afraid of losing her attention. In fanfiction, authors often play with the characters' emotions. Harry, on the other hand, might see Ron as being too possessive of Hermione.
One reason could be that they might have different interpretations of Harry's character in the fanfiction. Hermione, being logical, may see Harry in a more practical light, while Ron, with his emotional side, could have a different view.
One possible reason could be a misunderstanding. Maybe Harry overhears Ron and Hermione having a private conversation where they seem to be critical of him in some way, but they were actually just worried. Another reason might be if Harry starts to pursue a very different path in magic, like dark magic perhaps, and Ron and Hermione can't support or be associated with that. It could also be that a new character comes into their lives who tries to drive a wedge between them by spreading false rumors about each of them to the others.
One possible reason could be a divergence in their goals. Maybe in the fanfiction, Harry has a different approach to fighting Voldemort and Ron doesn't agree with it, leading to a rift. For example, Harry might want to take a more aggressive and risky approach while Ron prefers a more cautious one. This difference in ideology can drive them apart.
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.
Well, perhaps in the fanfiction they leave because of a prophecy. A prophecy might state that their presence near Hermione would bring great danger to her or to the wizarding world as a whole. So, they sacrifice their friendship and leave. Also, it could be due to some dark magic influence. They are forced by a powerful dark magic spell to stay away from Hermione, not having control over their actions.
It could be a misunderstanding. For example, Ron might think Harry did something really bad to Hermione or their other friends. Without proper communication, his anger could lead him to punch Harry. Also, if there was some sort of magical influence or dark magic at play that affected Ron's judgment, that could be a reason too.
Another reason could be their different views on how to raise their children. Hermione would likely be very strict with studies and following rules, while Ron might want a more laid - back approach. Disagreements over parenting styles can put a great strain on a marriage and could be a factor in their divorce.
One reason could be that Ron abandons Harry when he needs him the most. Like in a dangerous situation, if Ron runs away leaving Harry alone to face the threat, Harry would find it hard to forgive such cowardice.
Perhaps there were some secrets that Ron was keeping from Harry. In the wizarding world, secrets can be a big deal. If Ron had a secret related to his family or some other important matter and Harry found out about it in a way that made it seem like Ron was hiding something bad, it could lead to Harry not trusting him. Also, Ron's friendship with other characters might make Harry jealous or suspicious. If Ron was getting closer to someone Harry didn't trust, it could spill over and make Harry question Ron's loyalty to him.