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.
It could be due to the influence of other characters. Some characters in the fanfiction might plant seeds of doubt in Harry's mind about Dumbledore. They could spread rumors or share their own negative views of Dumbledore with Harry, causing Harry to start distrusting him. For instance, if a character tells Harry that Dumbledore has ulterior motives for training him, Harry might start to question Dumbledore's intentions.
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 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.
Another reason could be outside influence. If there were dark forces trying to drive a wedge between Harry and the Weasleys, they might plant false ideas in Harry's mind. This could make Harry question the Weasleys' loyalty, even though they are truly his friends.
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.
One possible reason could be that Harry discovers some secrets that Dumbledore has been keeping from him. Maybe Dumbledore has been making decisions about Harry's life without his knowledge, like how he was set up to face Voldemort. This lack of transparency might lead Harry to lose trust in him.
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.
One possible reason could be that Harry has grown tired of being Dumbledore's pawn. He might feel that he has been constantly pushed into dangerous situations for Dumbledore's grand plans and he just wants to focus on his own life and safety for once.
Maybe Harry finds out that Dumbledore knew more about his family's past than he let on. For instance, if Dumbledore had information about some dark secrets in the Potter family but didn't tell Harry, it could make Harry furious and prompt him to yell. Also, if Dumbledore's actions led to the death or harm of someone Harry cared about, like Sirius, Harry would likely be very angry with him.
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.