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.
It could also be due to outside influence. If there is a powerful dark force or a manipulative character in the fanfiction that convinces Harry that his parents are not what he thought they were. They might make him believe that his parents' actions led to some great harm that he can't overlook.
They could have been disowned if they took a stance on a political or magical issue that was against their parents' beliefs. For instance, if there was a conflict between pure - blood and half - blood wizards, and Harry and Ginny sided with the half - bloods when their pure - blood parents were firmly against it. This difference in ideology could cause a rift so large that their parents would disown them.
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 some false information was spread about Harry that made Sirius believe he had done something unforgivable. For example, if there were rumors that Harry was involved with dark magic in some way, Sirius, who has a strong aversion to the dark side, might be quick to disown him.
Well, it could be that Harry ends up in a new magical community that accepts him for who he is, away from the shadow of his disowning parents. He might learn new types of magic that were not taught in his previous life with his family. He could also meet a mentor figure who helps him through the tough times. And then, he might use his new - found knowledge and skills to face his parents again, but this time with confidence and self - assurance.
One possible plotline could be Harry feeling extremely lost and alone at first. He might have to find a new place to live, perhaps with the help of his friends like Ron and Hermione. He could struggle with self - worth, constantly wondering what he did wrong to make his parents disown him.
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 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 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.
Hermione would be a very likely character to support Harry. She's his best friend and is always there for him in the original story. Ron too, as they are a trio. Neville could also step up, as he has his own growth arc and could understand Harry's feeling of being alone to some extent.