One possible reason could be differences in their approach to fighting Voldemort. Sirius might be more reckless and Harry, having faced more direct threats, could be more cautious. Another reason could be Sirius' over - protectiveness of Harry. He might try to stop Harry from taking certain risks, which Harry sees as necessary for his growth and the defeat of the Dark Lord.
One reason could be Harry's impulsiveness. Harry often jumps into dangerous situations without thinking, and Sirius, being his godfather and having a protective nature, would shout at him to make him realize the risks. For instance, if Harry decides to go after a Death Eater alone.
One possible reason could be jealousy. For example, if Harry gets more attention in the wizarding world for his achievements, Ron might feel a bit left out and jealous, leading to an argument. Another reason could be differences in their approach to handling a dangerous situation. Ron might be more cautious while Harry could be more impulsive, and this clash of styles can cause them to argue.
One possible reason could be miscommunication. Maybe Harry does something that Sirius misinterprets as a betrayal or a stupid mistake, leading to Sirius lashing out.
One possible reason could be jealousy. Maybe Ginny sees Harry getting too friendly with another female character in the fanfiction world. Another reason might be differences in their future plans. For example, Harry may be too focused on his Auror duties and Ginny wants more family time.
One reason could be the strong bond between them in the original story. Fans might have felt that it could be developed into a father - son relationship in fanfic.
In some fanfictions, it could be due to dark magic influence. A spell or a dark force might have corrupted Sirius's mind, making him turn against Harry.
One reason could be some sort of ancient magical law or prophecy. Maybe there's a hidden clause that says for the greater good of the wizarding world, they need to be married. It could be related to some power or protection that their union might bring.
One reason could be for shock value. Writers might want to create something that stands out and grabs the attention of readers in a very different way. Another reason could be to explore a unique power dynamic between the two characters. Maybe the writer wants to show a different side of their relationship, like Sirius being in a more authoritative position over Harry in an extreme way.
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, one possible reason could be a dark magic spell that forces Harry to do such a thing against his will. Maybe a powerful dark wizard has put a compulsion charm on him. Another reason could be a misinterpretation of a prophecy that makes someone believe Harry must perform this act to prevent a greater evil. However, this is a very strange and dark concept for a fanfiction.