Well, perhaps in the fanfiction the Hogwarts staff might have been unfair in grading his exams. Like giving him lower marks than he deserved just because they had some sort of prejudice against him. It could also be that they wrongly accused him of something he didn't do, like misbehaving or cheating. Another reason could be that they failed to protect him or his friends from some sort of danger at Hogwarts.
One reason could be that he doesn't like others taking creative liberties with his story. He might feel that it's his story and only the original version should be the 'true' one.
In some fanfictions, there could be a change in the Hogwarts admission policy. Maybe they found some supposed flaw in Harry's magical abilities or his background that led to his admission being revoked at the last minute, so he didn't arrive at Hogwarts. Also, it's possible that some other powerful magic was at work. A powerful wizard or witch could have cast a spell on Harry that made him forget about Hogwarts or made it so that he couldn't physically reach the school.
Perhaps the wizarding government or authorities were being unjust. For example, if they wrongly accused Harry of a crime he didn't commit, his sense of justice would be outraged. His anger would drive him to fight back and prove his innocence. He's not the type to let false accusations stand.
Well, perhaps in the fanfiction, the Order made decisions without consulting Harry. They might have put him in a dangerous situation without his knowledge, like sending him on a mission that seemed suicidal. It could also be that they were keeping secrets from him that he felt were important to his fight against Voldemort.
In fanfiction, Harry Potter might be angry if his friends are in danger and no one seems to be taking it seriously. He's very protective of his friends, like Ron and Hermione. So if some authority figures in the story are being dismissive of the threats his friends face, that would surely make him angry.
One reason could be that he has fallen in love with someone outside of Hogwarts and wants to stay with that person instead. He may feel that Hogwarts will keep him away from his beloved.
Well, perhaps in the fanfiction, Harry has a vision of a different future for himself that doesn't involve traditional Hogwarts education. He might have seen a path where he can make a greater impact on the wizarding world without the constraints of school. It could also be that he's being influenced by a new character in the fanfiction, someone who convinces him that there are better ways to develop his magic and knowledge outside of Hogwarts. This new influence could be from a different magical culture or a rogue wizard with unique ideas.
One reason could be that it doesn't stay true to the original lore. For example, if the fanfiction changes the fundamental rules of magic in Hogwarts that were established in the original series. Another reason might be that the characters' personalities are distorted. Harry Potter is known for his courage and self - sacrifice, and if the fanfiction turns him into a cowardly or self - centered character, it could be rejected.
Perhaps he has seen a dark future for Hogwarts in a vision. He might think that by staying away, he can avoid that future or protect himself from some unknown danger. In fanfiction, there are often new and creative plotlines like this.
In fanfiction, the author might want to explore a different side of Harry. He could be berating Hogwarts education because he realizes that some students were left behind. There were students like Neville who didn't really shine until later, and Harry might think that the school system should have been more inclusive and better at nurturing all types of magic abilities from the start.