Maybe Harry uncovers evidence that Dumbledore had been manipulating him all along. Like Dumbledore knew more about Voldemort's connection to Harry than he let on, and he was using Harry as a pawn in a bigger game. Harry, feeling betrayed, decides to expose Dumbledore's true nature, which would shatter the positive image everyone had of him.
One possible motive could be that Harry discovers some long - hidden secrets about Dumbledore that he feels are unethical or immoral. For example, if he finds out that Dumbledore had some ulterior motives in his actions towards Harry throughout the years. Maybe Dumbledore was using Harry for a greater plan that involved a lot of sacrifice on Harry's part without his full knowledge or consent.
Well, perhaps Harry is under the influence of a dark spell that makes him lose control and lash out at Dumbledore's office. Maybe he discovers something in the office that he believes is a great betrayal by Dumbledore and in his rage, he starts to destroy things.
Some would be in shock and disbelief. People who idolized Dumbledore would initially refuse to believe Harry's claims. They would think Harry was either misinformed or had some malicious intent.
It could also be that Harry has a different vision for how to deal with the main threat in the wizarding world. Dumbledore's plans might seem too slow - paced or too conservative to Harry. He may believe that his own more direct and bold approach would be more effective, and thus decides to destroy Dumbledore's plans to implement his own.
It could be Voldemort. He might have found a way to manipulate Harry into attacking Dumbledore's office. By doing this, he not only causes chaos in Hogwarts but also tries to drive a wedge between Harry and Dumbledore.
One possible motive could be to explore a dark and alternate universe. The writer might want to see how the 'Harry Potter' world would change if such a drastic event occurred. Another reason could be to analyze the characters from a different perspective. By making Harry the destroyer of Dumbledore, it forces readers to re - evaluate both characters.
It shakes the foundation of the trust that fans have in the characters. Dumbledore is often seen as a wise and benevolent figure. If Harry destroys his reputation, it makes fans question all the previous events and decisions that Dumbledore was involved in. For example, his guidance to Harry might now seem suspect.
Perhaps Dumbledore is being forced in this fanfiction. There could be some dark magic or powerful dark wizard threatening Hogwarts or the wizarding world. Dumbledore, in a desperate attempt to save everyone else, gives Harry to Voldemort as a sacrifice. But Harry, being the hero he is, could find a way to turn the situation around and defeat Voldemort from within his lair.
It could be that there's a powerful enemy that has taken over Hogwarts and Harry, in a desperate attempt to stop this evil from spreading, resorts to destroying the school. He might think that rebuilding it later is better than allowing it to be used for evil purposes. For example, if Voldemort had found a way to permanently taint the magic of Hogwarts, Harry might see destruction as the only option.
One possibility could be that Harry discovers some dark magic or a hidden conspiracy within the Triwizard Tournament that forces him to take extreme measures and destroy it. Maybe he finds out that the tournament is being used as a cover for an evil plot against Hogwarts or the wizarding world in general.