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.
Perhaps in the fanfiction, there is a conflict over Harry's safety. Sirius is very protective of Harry, and if he feels that Harry is being reckless with his own safety in a way that Sirius deems unacceptable, he might kick Harry as a way of showing his extreme disapproval. Also, if there are secrets involved, and Harry accidentally discovers or reveals something that Sirius wanted to keep hidden, that could lead to such a violent reaction from Sirius.
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.
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.
Well, it's a rather unusual concept for fanfiction. Usually, Sirius Black is more of a protector or mentor figure to Harry Potter. However, if you search on platforms where Potter fanfictions are hosted, you might find some that explore this idea in a creative way. Maybe it's part of an alternate universe scenario where their relationship takes a different turn for some plot - driven reasons.
A common reason could be misinformation. In the wizarding world, rumors can spread quickly. If a false rumor about Harry being involved in dark magic activities got to Sirius, he might disown him. For example, if someone planted evidence that Harry was secretly communicating with Death Eaters.
One possible reason could be that in the fanfiction, the wizarding community in England has turned against Harry due to some false accusations. Maybe a powerful dark wizard has framed him for a series of heinous crimes, and the public, being misled, forces him to leave. Another reason might be political intrigue within the Ministry of Magic. There could be a new regime that sees Harry as a threat to their power and thus kicks him out.
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.
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.
Well, if Sirius Black hates Harry Potter fanfiction, it could lead to a divide among fans. Some fans might respect his view and be more critical of fanfiction themselves, while others might disregard his feelings. Also, it could potentially limit the creativity within the fan community as people might be afraid to create content that could be seen as disrespectful to Sirius Black's character.
One reason could be the strong bond between them in the original story. Fanfic writers might think that making them father - son would enhance that bond further. Also, Sirius was a very important figure in Harry's life, so it makes for an interesting twist.
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.