Another reason could be related to a love triangle situation. If there is a girl involved and Ron and Harry both have feelings for her, and Ron does something underhanded to win her over, like spreading lies about Harry, it would be understandable for Harry to start hating Ron.
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.
One reason could be a divergence from the original character traits. In the original story, Hermione and Ron have a complex relationship with love and friendship. Fanfiction that misrepresents Hermione's feelings towards Ron might go against the essence of her character. Maybe the fanfiction portrays her as hating Ron in a way that doesn't fit with her growth and development in the official series.
Maybe in that fanfic, there are misunderstandings between Harry and the Weasleys. For example, they might have had an argument over something like magic use or family values that led Harry to dislike them.
One reason could be a divergence from the original friendship dynamic in the books. In the canon, Harry and Ron are good friends, and fanfiction that shows Harry hating Ron might go against what many fans consider the essence of their relationship.
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 reason could be a miscommunication. Maybe Hermione accidentally reveals a secret of Harry's that he didn't want known, and this causes him to feel betrayed and start to hate her. Another reason could be jealousy. If Hermione gets more attention or praise for her magical abilities than Harry in the fanfic, he might develop feelings of envy and hatred. Or perhaps there's a love triangle situation where Hermione is involved with someone Harry doesn't like, and that causes his hatred.
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.
One possible reason could be a misinterpretation of his character. In the original series, Harry Potter doesn't hate Muggles at all. But in this fanfic, perhaps the author wants to explore a 'what if' scenario where something drastic has happened to make him have such feelings. Maybe he's been betrayed by a Muggle - in the fanfic world, this could be a made - up situation that sours his view on all Muggles.
Perhaps in the 'harry hates dunbledore fanfiction', Dumbledore is shown as being involved in some sort of secret plot that goes against Harry's values. For example, if Dumbledore was secretly working with the dark side in the fanfiction, Harry would naturally hate him. Also, it could be that Harry has discovered some long - hidden truth about Dumbledore that completely changes his view of him, like a past misdeed that Dumbledore has covered up.
Maybe in that fanfiction, McGonagall has some actions that Harry misinterprets. For example, she might be too strict with him in a way that he can't understand at that moment.
It could be due to a misunderstanding. In the fanfic, Remus might say or do something that Harry misinterprets as being against him. For example, Remus could be trying to protect Harry but his actions come across as controlling, which would make Harry dislike him and potentially hate him.