Maybe Harry is being influenced by someone or something that has made him distrust Dumbledore. It could be a new character in the fanfiction who has convinced Harry that Dumbledore has ulterior motives. So, in order to protect himself or keep his new - found knowledge or power a secret, he hides.
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.
Another reason could be that Harry has made a mistake or broken a rule that he fears Dumbledore will punish him severely for. He might think that by hiding, he can figure out a way to fix the situation on his own before facing Dumbledore.
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 fear. Maybe Harry discovers something about Dumbledore in the fanfiction that makes him scared, like a hidden agenda. Another reason might be to protect Dumbledore. Harry could think that by hiding from him, he is keeping Dumbledore away from some sort of danger that is related to Harry himself. Also, it could be due to a misunderstanding. Harry might misinterpret something Dumbledore does or says, and as a result, decides to hide from 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.
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.
Well, it could be that Harry has a different view on how to handle the war against Voldemort. Dumbledore is often very strategic and sometimes keeps things from Harry for what he believes is the greater good. But Harry might think that full transparency is necessary. Also, Harry may have formed new alliances or friendships that give him different perspectives, and these new ideas conflict with Dumbledore's. He might feel that Dumbledore's approach is too old - fashioned or risky in a new context.
Perhaps there are safety concerns for the students that Dumbledore is not addressing adequately. Harry, having a strong sense of responsibility for his fellow students, may feel that the only way to ensure their safety is to remove Dumbledore from his position. He might believe that Dumbledore's actions or inactions are putting the students at risk, and thus he takes matters into his own hands.
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 a fanfiction, Dumbledore might steal out of desperation. Suppose there was a situation where a curse was about to be unleashed on Hogwarts, and the only way to stop it was by stealing a particular ingredient for an antidote from a place where it was not supposed to be taken. Dumbledore, seeing no other option, decides to steal it. His motive here is to save the school and its students, even if it means breaking the rules.
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.
One reason could be a change in Dumbledore's personality. If he's depicted as being cold or distant in fanfiction, Harry, who knew him as a mentor figure, might not like it.