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.
Power struggle could also be a factor. Harry, as he grows more powerful and independent, might start to question Dumbledore's authority and the way he wields power within the wizarding world. He may think Dumbledore is being too controlling and wants to break free, thus creating an 'against' situation.
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.
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.
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.
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 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 reason could be a difference in ideology. Harry might have come to believe that Dumbledore's methods, like keeping secrets from him, are not in the best interest of the wizarding world. For example, Dumbledore withheld information about Harry's connection to Voldemort which put Harry in more danger at times. Harry could stand up to him to demand more transparency.
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.
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 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.
I'm not sure if there is a specific 'Harry Potter Dumbledore Stealing Fanfiction' out there. However, if there were, the plot could potentially involve Dumbledore stealing something important like a powerful magical artifact that he believes is necessary to protect Hogwarts or the wizarding world from a great evil, but his actions are misinterpreted by the characters in the fanfiction.