Maybe in this fanfiction, Harry has been influenced by some dark forces or characters who convince him that his parents' actions led to a series of bad events in the wizarding world. So, he decides to disown them to break free from what he sees as their negative legacy.
It could also be that the fanfiction writer wants to explore a different side of Harry's character. By having him disown his parents, it shows a more rebellious and independent Harry. He might feel that the expectations placed on him because of his parents are too much to bear, and so he takes this extreme step to define himself on his own terms.
One possible reason could be that in the fanfiction, Harry might have discovered some dark secrets about his parents that made him feel betrayed. For example, if he found out they had some hidden and unethical magical experiments in their past.
Well, it might start with Harry having doubts about his parents. Then, as he digs deeper into their past, he could start to distance himself. Eventually, he publicly disowns them, which would shock the wizarding community. This could lead to him being ostracized by some and supported by others who also have doubts about his parents' actions.
One possible reason could be that he might feel abandoned. Growing up without them, he could resent the fact that they left him an orphan. He might think that if they were really so great, they would have found a way to survive and be there for him.
One possible reason could be that he discovers some long - hidden family secrets that make him question his parents' actions or decisions. Maybe he finds out about some dark deeds in their past that he can't overlook.
One possible reason could be a misunderstanding. Maybe he discovers some secrets about his parents that he misinterprets. For example, if he finds out they were involved in some dangerous or morally ambiguous activities in their youth that he doesn't fully understand at first.
Maybe they were forced into it by some dark magic or a powerful dark wizard. They could have been blackmailed into betraying Harry to save their own lives or the lives of others they cared about.
One possible reason could be to explore different parenting styles in the wizarding world. Spanking, although a controversial form of discipline, might be used by some writers to show how Harry's parents could have dealt with difficult situations. Another reason might be for shock value or to create a unique and unexpected story within the familiar Harry Potter setting. It could also be a way for the writer to delve deeper into the characters' personalities and how they might react in a situation that involves discipline.
Perhaps he discovers some secrets about his parents that he doesn't like. For example, he might find out that they were involved in some dangerous or unethical magic before. This new knowledge could make him angry at them for putting themselves and their family in jeopardy.
One possible reason could be that in the fanfic, the author might twist the story to make it seem like his parents' actions had unforeseen consequences that affected Harry negatively. For example, their fight against Voldemort led to Harry being an orphan and having to bear a great burden from a young age.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.