One possible reason could be a difference in ideology. Maybe Harry starts to question Dumbledore's decisions regarding the war against Voldemort. For example, Dumbledore's secrecy about certain things might make Harry feel left out and manipulated. Another reason could be personal pride. Harry has grown into a powerful wizard and may not want to be under Dumbledore's constant guidance and control any longer. He might think he can handle things on his own better. Also, it could be due to the influence of other characters. If some new friends or allies have different views on Dumbledore, they could sway Harry's opinion.
The change in the storyline would be multi - faceted. Emotionally, Harry would be on a different journey. He would be dealing with feelings of betrayal (if he believes Dumbledore has wronged him) and determination to prove himself right. In terms of the plot, the search for Horcruxes might take a different turn. Harry might not follow Dumbledore's clues as closely. He could end up finding Horcruxes in different ways or even missing some if he doesn't have Dumbledore's wisdom. And the final confrontation with Voldemort would be more unpredictable. Harry's rebellion could lead to a more evenly matched battle as he might not have all the advantages that Dumbledore's guidance would have provided.
One reason could be a misunderstanding. Maybe Harry discovers some actions of Dumbledore that seem wrong at first glance, like the way he's been handling secrets about Harry's past or Hogwarts' safety. For example, Dumbledore's decision to keep Harry in the dark about certain prophecies might make Harry feel betrayed and lead to a conflict.
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.
In the Harry Potter series, saying that Dumbledore "schemed" against Harry Potter was a bit biased. Dumbledore did have a plan for Harry's growth and destiny in many ways. For example, he gave Harry to the Dursleys to be raised. On the one hand, it was to keep Harry away from the danger of being tainted by fame and fortune at an early age. Growing up in an ordinary family in the Muggle world, Harry grew up in such a repressive environment and had a strong ability to resist pressure. On the other hand, it was to protect Harry from Voldemort or his henchmen. Throughout the entire plan against Voldemort, Dumbledore knew Harry's special identity and his key role in the fight against Voldemort. Although he knew that Harry might face great danger or even death, he was trying his best to guide, support, and protect Harry. He had always been Harry's mentor and supporter, helping Harry explore the wizarding world, unravel his family's secrets, and uncover the truth behind Voldemort's schemes. His arrangements were more out of the grand goal of protecting the Wizarding world, fighting against Voldemort, and the comprehensive consideration of Harry's growth and development, rather than malicious " calculations." Harry Potter and the Order of the Phoenix is not enough. Please click to read the novel!
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.
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 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.
Fanfiction often creates alternate scenarios. It could be that Dumbledore has a hidden agenda that Harry uncovers, which goes against Harry's sense of right and wrong. Maybe Dumbledore is trying to sacrifice Harry in a way that Harry doesn't agree with, leading to their opposition. This can be a very creative way to rewrite the dynamics between the two characters.
One reason could be secrecy. If Harry finds out that Dumbledore has been keeping major secrets from him all along, like about his family or his true role in the wizarding world, he might start to hate him. Another reason could be manipulation. If it seems that Dumbledore has been manipulating Harry's actions for his own gain, that would be a valid cause for hatred in fanfiction.
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.