Another reason might be a misunderstanding. Ron could misinterpret Harry's actions or words in a way that makes him think Harry has betrayed him first. So, in his anger and hurt, he decides to retaliate by betraying Harry. It's a common trope in fanfictions to play on misunderstandings between characters to create drama.
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.
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.
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.
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.
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.
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.
One possible plotline could be that Ron and Hermione are influenced by dark magic or a powerful enemy who manipulates them into betraying Harry. For example, they might be under the Imperius Curse. Another could be that they misunderstand Harry's actions or intentions, leading them to believe he has gone to the dark side, so they turn against him in an attempt to 'save' the wizarding world. Maybe they are jealous of Harry's fame and power growing too much, and this envy festers until they decide to betray him.
One possible reason could be a plot device to create drama. By having Harry ignore Ron, it can lead to tension within their friendship, which is a central relationship in the original series. This can then drive the story forward as they have to resolve this issue.
Another reason could be a misunderstanding. Maybe he overhears something Harry and Hermione say and misinterprets it as them not wanting him around anymore. So, in a fit of anger and hurt, he leaves. This can lead to a lot of angst and drama in the fanfiction as the other characters then have to figure out how to get him back.
Maybe they were forced into it by some dark magic or a powerful dark wizard. They could have been blackmailed into betraying Harry to save their own lives or the lives of others they cared about.
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.