One possible reason could be that she's frustrated with the school's bureaucracy. For example, if she constantly faces obstacles in getting approval for new study groups or advanced classes. Another reason might be a personal quest. Maybe she discovers a hidden mystery outside of Hogwarts that she can't explore while being a student there.
She could leave Hogwarts in a fanfic because of a falling - out with a teacher. Suppose she has a major disagreement with a professor over a new teaching method that she believes is harmful to the students' learning. Or perhaps she feels that she has outgrown what Hogwarts can offer her academically and wants to seek more advanced knowledge elsewhere.
One reason could be that she might be on a secret mission. In some fanfictions, she could be trying to find a powerful magical artifact outside the walls of Hogwarts to save the wizarding world from a new dark threat. She might believe that she can't achieve this while staying within the school's confines.
Maybe she leaves because of a conflict. It could be with a teacher who doesn't support her unorthodox ways of learning or a fellow student who makes Hogwarts an unpleasant place for her. In fanfiction, there could be a lot of creative interpretations of such conflicts.
One possible reason could be a creative plot twist by the fanfic writer. Maybe they want to explore how Harry would grow and develop on his own without Hermione's support. Another reason might be to introduce new and different story arcs that involve separation, like Hermione having to leave for a dangerous mission that she can't involve Harry in.
Well, her friends would surely miss her a great deal. Harry and Ron would feel like they've lost an important part of their team. Academically, the Gryffindor house might see a decline in grades as Hermione was often the one helping others study. Also, the teachers might be disappointed as she was a brilliant student.
Perhaps in the fanfiction, Hermione leaves Ron because of his jealousy issues. Ron has shown bouts of jealousy in the original series, and if that continued to be a problem in the fan - created story, it could be a major factor. For example, if Hermione started to gain more recognition in the wizarding community for her intelligence and contributions, and Ron couldn't handle it well. Or it could be that new characters were introduced in the fanfiction, and Hermione developed a stronger connection or friendship with them, which made her re - evaluate her relationship with Ron.
One possible reason could be for adventure. Hermione is a very curious character. Maybe in the fanfiction, she has heard of some mysterious magical place outside England that she just has to explore. For example, there could be a hidden library full of ancient and powerful spells that is only accessible in a far - off land.
Maybe the fanfiction portrays some really dark and disturbing events that make Harry feel he can't stay at Hogwarts anymore. For example, if it involves his friends being seriously harmed in the fictional story in a way that he can't bear to be associated with the place where it all seems to be happening.
One reason could be to search for a powerful magical artifact outside of Hogwarts that they believe is crucial for defeating a new dark threat. In fanfiction, they might feel that Hogwarts has become too dangerous or restricted in terms of resources for their mission.
One reason could be she feels extremely pressured by the high expectations at Hogwarts. Everyone expects her to be perfect in academics and problem - solving. Another might be that she discovers some dark secret within the school that she's afraid of, like a hidden plot by a group of dark wizards among the staff or students. Or she could be running away to protect her friends. If she believes that her presence at Hogwarts endangers Harry and Ron, she might choose to leave to keep them safe.
One possible reason could be a misunderstanding. Maybe Hermione overhears something Harry says and misinterprets it. For example, if Harry is discussing some dangerous plan with Ron and Hermione thinks he's being reckless without considering her input, she might leave in anger. Another reason could be her own personal growth. She may feel that she needs to explore her own magical abilities further on her own and that Harry's presence is somehow holding her back in that regard.