Perhaps in these fanfictions, Dumbledore is constantly portrayed as all - knowing and all - powerful, which could frustrate Harry. For example, if Dumbledore is always solving problems before Harry has a chance to grow and figure things out on his own. Harry, being a strong - willed character, would naturally have enough of this situation.
One reason could be that in some fanfictions, Dumbledore's character might be over - idealized or his actions overly scrutinized in ways that make Harry seem secondary. Harry might be tired of always being in Dumbledore's shadow or having Dumbledore's decisions dictating his life even in the fan - created stories.
It could also be that the fanfictions keep repeating the same tropes regarding Dumbledore and Harry's relationship. Like Dumbledore always being the one to give cryptic advice and Harry having to blindly follow it. After a while, this can become tiresome for Harry's character in the fanfiction world, making him seem like he has no agency of his own.
Maybe Harry Potter is tired of the unrealistic portrayals of Snape in fanfiction. Often fanfictions can exaggerate characters' traits, and Harry, who knew Snape in real life, might find these interpretations too far - fetched.
One possible reason could be a misunderstanding. Maybe Harry overheard something that made him think Dumbledore was planning something against him that he didn't understand. For example, if he heard Dumbledore discussing a dangerous mission for him without his full knowledge.
One possible reason could be that Harry has discovered something about himself or his past that he thinks Dumbledore might not approve of or might try to stop him from exploring further. For example, if he found out some dark magic connection in his family history that he wants to understand on his own without Dumbledore interfering.
It could be that Harry finds out about Dumbledore's secrets regarding the Deathly Hallows. If Harry thinks Dumbledore was more interested in the Hallows for his own purposes rather than for the greater good, it might make him hate Dumbledore.
A possible reason could be jealousy. Maybe Harry sees Dumbledore getting all the praise and respect while he is the one doing all the dangerous work. So, he decides to expose some of Dumbledore's flaws to get some of that attention back.
Maybe Harry discovers some things from Dumbledore's past that seem unethical to him. For example, Dumbledore's association with Grindelwald in his youth. Harry, who has a strong sense of right and wrong, might find it hard to trust someone with such a controversial past. And in the fanfiction, the author might explore how these revelations shake Harry's faith in Dumbledore.
One possible reason could be that Harry breaks school rules in the fanfiction. For example, if he ventures into the Forbidden Forest without permission. Dumbledore is strict about the safety of students and such disobedience might lead to punishment.
Perhaps there are safety concerns for the students that Dumbledore is not addressing adequately. Harry, having a strong sense of responsibility for his fellow students, may feel that the only way to ensure their safety is to remove Dumbledore from his position. He might believe that Dumbledore's actions or inactions are putting the students at risk, and thus he takes matters into his own hands.
Well, it could be that Harry has a different view on how to handle the war against Voldemort. Dumbledore is often very strategic and sometimes keeps things from Harry for what he believes is the greater good. But Harry might think that full transparency is necessary. Also, Harry may have formed new alliances or friendships that give him different perspectives, and these new ideas conflict with Dumbledore's. He might feel that Dumbledore's approach is too old - fashioned or risky in a new context.
A possible reason is that Harry feels betrayed. If he discovers that Dumbledore has been using him as a pawn in a bigger game against Voldemort without his full knowledge or consent, he might want to get back at Dumbledore. For instance, if he finds out that Dumbledore knew about certain risks to his life but still pushed him into dangerous situations, he could decide to turn against Dumbledore.