In such a fanfiction, one possibility could be that Harry Potter defies Dumbledore over the use of certain dark magic items. Maybe Harry believes that Dumbledore is being too cautious and that these items could be used for good in the fight against Voldemort. For example, he might think that the Elder Wand could be harnessed in a different way than Dumbledore anticipates.
One possible scenario could be that Harry feels betrayed by Dumbledore for some reason. Maybe he discovers some secrets that Dumbledore has been keeping from him, like the true nature of his family's past or the real intentions behind certain events at Hogwarts. So, in a fit of anger and disillusionment, he decides to ignore Dumbledore.
The Weasley family as a whole might also support Harry. They have always treated Harry like one of their own. Arthur Weasley, being an open - minded wizard, might understand Harry's perspective. Ginny, who is very close to Harry, would surely be on his side as well. They would see Harry as more than just a student defying a headmaster, but as someone fighting for what he believes is right.
One possible scenario is when he meets an ancient and powerful magical being that has just emerged after a long slumber. Dumbledore, in his wisdom, bows to show respect for this being's age - old magic and power. Another could be in a situation where he is facing a group of powerful wizards from a different culture with their own traditions, and he bows as a sign of diplomatic respect.
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 possible scenario could be that Dumbledore is giving Harry some important advice in a very intimate and fatherly way. Harry might be feeling sad or confused and sitting on Dumbledore's lap makes him feel safe and listened to.
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, one possibility could be Itachi as a wizard in the Harry Potter world. Maybe he'd be in Slytherin due to his cunning nature. He could use his Sharingan abilities in a magical way, like seeing through invisibility cloaks or detecting hidden spells.
It could be that after a series of events that make Harry question his own judgment, he submits to Dumbledore's authority. For instance, if there are false prophecies or misinformation floating around that lead Harry astray. Dumbledore, with his greater experience and knowledge, helps Harry see the truth. So Harry decides to submit to Dumbledore in order to get back on the right track and protect Hogwarts and the wizarding world.
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.
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.