It might be that Harry has a different vision for Hogwarts. He thinks Dumbledore's old - fashioned ways are holding the school back. For example, Dumbledore might be too cautious about teaching certain types of magic that Harry believes are essential for students to learn in order to defend themselves against new threats. So Harry decides to sack him to bring about radical change at Hogwarts.
Well, it could be that Harry discovers some dark secrets about Dumbledore's past actions. Maybe Dumbledore made a deal with some dark forces long ago that Harry thinks endangers the wizarding world. So, in a fit of moral outrage, Harry decides to 'sack' him, which could mean removing him from his position of power and influence at Hogwarts.
Perhaps Harry has been misinformed. There could be a character who spreads false rumors about Dumbledore. Harry, being young and impulsive, believes these rumors without proper investigation. He then takes actions to sack Dumbledore. Later on, he realizes his mistake and has to make amends.
Hermione might support Harry if she also sees some flaws in Dumbledore's leadership. She is very logical and if Harry presents her with solid evidence, she could side with him. For instance, if Dumbledore's actions are putting students in danger in some way, Hermione would want to protect her fellow students.
Well, perhaps there's a fanfic where Dumbledore travels to the Muggle world for some reason and stumbles upon a baseball game. He gets hold of a baseball bat and realizes it has some interesting properties. Back at Hogwarts, he uses magic to enhance the bat and it becomes a sort of magical weapon that can be used against dark forces. Maybe it has the power to deflect dark spells or even send out bursts of positive energy. And the students are both amused and amazed by this new addition to Dumbledore's arsenal.
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.
In this fanfic, Dumbledore might de - age Harry to give him a second chance at childhood. Harry had a tough childhood, and Dumbledore could think that by de - aging him, he can give Harry a chance to experience a more normal and happy childhood. This would lead to a lot of character development as Harry would have to deal with his past traumas while being in a child's body, and he might also discover new things about his magic along the way.
One possible plot could be that Harry discovers a powerful and ancient magic that Dumbledore has been trying to keep hidden. Harry's newfound knowledge and power make Dumbledore fear that he will disrupt the delicate balance in the wizarding world. So, Dumbledore starts to act nervously around Harry and tries to subtly keep him in check.
One possible plot could be that Harry discovers some dark secret about Dumbledore's past actions and decides to oppose him. For example, if he finds out that Dumbledore had some sort of deal with the Dark Arts that he hid from everyone. This would make Harry feel betrayed and drive him to defy Dumbledore.
Well, perhaps Harry discovers some dark secret about Dumbledore that makes him feel he must escape. It could be that Dumbledore is being controlled by some dark magic and Harry realizes he's in danger around him.
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.
In such a fanfic, perhaps Harry discovers some long - hidden, damning secret about Dumbledore. Maybe it's related to Dumbledore's past actions with Grindelwald. Harry, being the honest and justice - seeking person he is, feels compelled to expose this secret, which then leads to Dumbledore being fired from his position at Hogwarts.
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.