It could also be a matter of character development in the fanfic. The writer might want to explore a different side of Harry, a more rebellious and distrustful side. So, they create a scenario where Harry denounces Dumbledore as a way to break away from the traditional view of their relationship and show how Harry can be independent in his thinking.
Perhaps in the fanfic, there's a new plot device introduced. Maybe a dark magic influence has made Harry believe that Dumbledore is actually working against the greater good. This could be a new evil spell or a manipulative character who has poisoned Harry's mind against Dumbledore.
Maybe Dumbledore discovers some dark secret about Harry's past that he can't handle. It could also be that Dumbledore is trying to protect Harry in a really strange way, by pushing him away from the dangerous wizarding world around Hogwarts.
There are several possible reasons. Maybe Harry discovers a powerful and dangerous magical artifact and starts toying with it without proper guidance, which Dumbledore sees as a huge risk. Or perhaps Harry spreads rumors or secrets that Dumbledore has entrusted to him, and Dumbledore feels the need to discipline him to protect the safety and stability of Hogwarts. Fanfic authors often use such situations to show the complex relationship between Dumbledore and Harry and how they navigate through difficulties.
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.
One possible reason could be that Dumbledore believes he is acting in Harry's greater good. Maybe he thinks that by taking certain items from Harry, he can protect them from falling into the wrong hands. For example, if Harry has a powerful magical artifact that Voldemort desires, Dumbledore might steal it to keep it safe.
One possible reason could be if Harry were to break a major school rule in the fanfic, like using dark magic without proper authorization. Dumbledore is a strict adherent to the rules of Hogwarts and would likely punish such behavior.
Perhaps in the fanfic, Harry discovers that Dumbledore has been manipulating him all along. Dumbledore has always been a very complex character with many secrets. If Harry were to find out that Dumbledore was using him as a pawn in a much larger and dangerous game, his anger could lead him to punch Dumbledore. This would completely change the dynamic between them and open up a whole new set of storylines and character developments.
One possible reason could be that Harry discovers some secrets about Dumbledore that make him lose trust. For example, if he finds out about Dumbledore's past mistakes or hidden agendas that he doesn't agree with. Another reason might be that Harry feels he can better protect his friends and fight Voldemort on his own terms, away from Dumbledore's influence and strategies which he may start to question.
One possible reason could be a misunderstanding. Maybe Harry is led to believe that Dumbledore has been hiding something crucial about his parents' death or Voldemort's return. For example, if Harry discovers some encrypted files in Dumbledore's office that seem to suggest Dumbledore had a prior deal with Voldemort, it could lead to a fight. Another aspect could be related to power. If Harry feels that Dumbledore is being too controlling over his actions and the use of his powers, he might lash out. In the wizarding world, power struggles are not uncommon, and Harry, as a powerful wizard in his own right, might want to assert his independence.
One possible reason could be that Dumbledore wants Harry to have a more normal upbringing away from the spotlight of Hogwarts and his own influence. So he deliberately steps back from the role of guardian.
One reason could be that Dumbledore had too many other responsibilities in the wizarding world. He was the headmaster of Hogwarts and was constantly involved in dealing with the threat of Voldemort on a larger scale. He might not have been able to give Harry the individual attention and care that a true guardian should provide.