Another reason could be related to his connection with Voldemort. Some fanfiction authors might explore the idea that the school is afraid of the potential danger Harry brings because of his link to Voldemort. They might think that by expelling Harry, they can keep the school safe from any potential attacks or influence from Voldemort's remaining followers.
Perhaps he breaks a major school rule in a way that the school can't overlook. For example, if he accidentally endangers the lives of other students while trying to fight off a new dark threat within the school grounds. This would put the school in a difficult position and they might choose to expel him to maintain order.
One possible reason could be that he is wrongly accused of using dark magic. In some fanfictions, the Ministry of Magic might be more corrupt or easily influenced, and they could frame Harry for something he didn't do, leading to his expulsion.
One reason could be false accusations. Maybe someone frames Jaune for a serious offense he didn't commit, like cheating in an important exam or stealing valuable items from the school. Another reason might be his inability to keep up with the academic or combat requirements. If he fails too many classes or consistently underperforms in combat training, the school might decide to expel him.
One possible reason could be that he was framed for a serious offense he didn't commit. Maybe someone planted false evidence of him attacking a fellow student. Another reason might be that he continuously defied the school's strict rules in order to pursue his own goals of getting stronger faster, which the school authorities couldn't tolerate.
He might be extremely angry and feel a great sense of injustice. He has always seen Hogwarts as his true home, and being expelled would be like losing his family. So he would probably fight back, trying to prove his innocence and get his expulsion reversed.
In some fanfictions, Harry might be expelled for being wrongly accused of using dark magic. Maybe someone framed him by planting dark magic items in his dorm. Another reason could be that he continuously breaks school rules in a way that the school can't tolerate, like sneaking into the Forbidden Forest too many times without permission.
One reason could be that he has a change in motivation. Maybe he realizes that good grades can help him achieve his long - term goals, like becoming an Auror. So he starts to put in more effort into his studies.
Perhaps Harry accidentally unleashes a powerful and uncontrollable magic spell within the school grounds that causes a great deal of damage. Despite it being an accident, the Hogwarts administration, fearing further chaos, decides to expel him. Harry would then have to go on a journey to learn more about this magic and how to control it, all while trying to prove to Hogwarts that he is worthy of being readmitted.
One reason could be to explore the darker side of human nature. Even though Harry is a hero in the original series, fanfiction writers might want to see how he would be affected if he was pushed to his limits. Another reason is to create a more complex and controversial storyline. It grabs readers' attention as it goes against the normal perception of Harry.
Some fanfiction writers may have a different vision for Harry Potter's character. They might see him as more of a diplomat or a healer within the wizarding world, rather than a fighter. So, they write stories that reflect this different view of the character, where he uses his wits and other skills rather than engaging in physical or magical combat.
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.