In the fanfic, there might be a fight between them over a misunderstanding. Maybe there's a rumor about Harry being a dark wizard or something like that, and Ron, believing it for a moment, confronts Harry. The argument turns physical, and in the heat of the moment, Harry breaks Ron's arm.
It could be due to a spell gone wrong. Suppose Harry is trying out a new spell he's not fully mastered yet, and Ron is in the wrong place at the wrong time. Harry casts the spell, and it backfires, resulting in Ron's broken arm.
Maybe in the fanfic, Harry could break Ron's arm accidentally during a Quidditch match. For example, if their brooms collide harshly and Harry loses control, he might accidentally hit Ron and cause such an injury.
The fanfic might be set in an alternate universe where Ron has a different personality. If this new personality clashes with Harry's values or his own personality, then Harry may not like Ron. For example, if Ron is made to be more self - centered in this fanfic, Harry might find it hard to get along with him.
Maybe in the fanfic, there could be a misunderstanding between them. For example, Ron might have accidentally broken Harry's trust in some way, like revealing a secret of Harry's that he wasn't supposed to.
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.
In the fanfic, perhaps there was some deep - seated jealousy in Ron. He might have felt overshadowed by Hermione's intelligence and Harry's fame for a long time, and it all just boiled over in this really wrong way.
It could be that they lied to Harry about something really significant. In the wizarding world, trust is built on truth - telling. If they told a big lie, like about the true nature of a magical object that was putting Harry in danger, he would be very angry. And if their actions led to the death of someone Harry cared about. For instance, if they made a wrong move during a fight that caused an ally to die, Harry would find it very hard to forgive them.
One reason could be that the fanfic writer portrays Ron as overly jealous of Harry. In the original books, Ron has moments of jealousy but not to an extreme. If in the fanfic it's overdone, Harry might not be able to stand him. For example, if Ron is constantly trying to undermine Harry's achievements out of jealousy.
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 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.
Well, it could be related to Ron's insecurities. He might be insecure about his place in the group with Harry and Hermione. Hermione is always so confident and sure of herself, and Ron might feel that he doesn't measure up. This could translate into aggression as a way of lashing out. Also, perhaps there's some external influence, like a dark magic object that's affecting Ron's mood and behavior towards Hermione.
In the fanfic, Hermione might regret choosing Ron over Harry because Harry has a different level of understanding when it comes to her academic pursuits. Harry, having faced the challenges of fighting Voldemort from a young age, might have a deeper respect for Hermione's thirst for knowledge and her need to constantly learn and improve. Ron, on the other hand, sometimes showed impatience with Hermione's studious nature.