It could also be that Harry has been influenced by other characters. For instance, if some of his friends have different views on Dumbledore and convince Harry that he should not blindly listen to him. There might be a situation where Harry realizes that Dumbledore's way may not always be the best way to deal with the threats they face in the wizarding world.
Perhaps Harry has grown tired of always following Dumbledore's instructions without fully understanding the reasons behind them. He might want to make his own decisions and take control of his own destiny, rather than relying on Dumbledore all the time.
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 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 he has been through so much in his original adventures that he's just tired. He might feel like he's constantly being put in new, often contrived, situations in the fanfiction and he simply wants to step away from all the chaos and drama.
Well, it would change the power dynamic. Usually, Dumbledore is seen as a guiding figure for Harry. If Harry stops listening, he might make more mistakes as he lacks Dumbledore's wisdom. For example, in battles, he might not make the strategic decisions that Dumbledore would have advised.
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.
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 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 that Harry might feel he's been used too much in the past by Dumbledore. He could be tired of being thrust into dangerous situations without having a full understanding of the whole plan. For example, in the original series, he often felt like he was just following Dumbledore's orders blindly and it led to great personal risk and the loss of some friends. So in the fanfic, he might have decided enough was enough.
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.