Well, it could be for love. In many fanfics, their relationship has been through a lot, and leaving together is a way to start anew, away from the prying eyes and pressures of Hogwarts and the wizarding community.
Hermione could leave Draco if she realizes that they have fundamental differences in values. For example, if Draco still holds some pure - blood supremacist views, Hermione, being a Muggle - born advocate, would not be able to tolerate it in the long run.
It might be because of a misunderstanding. In some fanfics, a false rumor could spread about Harry. Ron and Hermione, believing it to be true, might decide to distance themselves from him. Say, there's a rumor that Harry is involved with dark magic in some way, and they, being the 'good guys', think they need to stay away.
Perhaps she tries to find them right away. Using her intelligence and resourcefulness, she starts to search for any clues as to where they might have gone. She could ask other friends, like Neville or Luna, if they have seen Harry and Ron. And she might be determined to bring them back, not willing to give up on their friendship easily.
One reason could be that Sirius represents a sense of freedom and rebellion that Hermione finds appealing. He's been through a lot and has a certain non - conformist attitude. In contrast, Ron can be a bit more traditional in his views at times.
In fanfiction, Hermione might choose Harry over Ron because Harry often has a more heroic image. He is the Boy Who Lived, and his adventures are often more central to the main plot. Hermione could be attracted to his leadership qualities and the fact that he always seems to be at the heart of the action.
In fanfiction, Ron might be portrayed as possessive of Hermione because in the original series, he has deep feelings for her. He may fear losing her to others, like Harry or new characters in the fan - created scenarios. His insecurities about their relationship could lead to possessive behavior.
Maybe it's because he has deep, unexpressed feelings for Hermione in the original story. Fanfiction allows him to be in different scenarios with her that he might have secretly desired in the real plot.
In fanfiction, it could be that Charlie has a more mature understanding of Hermione's intellectual pursuits. Ron sometimes struggles to keep up with Hermione's thirst for knowledge. Charlie, with his own expertise in his field, might be able to engage Hermione in more in - depth conversations, leading her to consider a relationship with him instead of Ron.
Well, some might write it to explore alternative character relationships. Maybe they think Hermione could have a better connection with someone else in the wizarding world, like Harry perhaps. It could also be a way to break free from the traditional canon pairing and create a more unique story.
There's often the sacrifice plotline. They might sacrifice their relationship with Hermione to save a large number of people. For instance, they learn that a dark wizard will spare many lives if they abandon Hermione. Also, the jealousy plotline can exist. If a new character becomes closer to Hermione and they feel left out, they might leave out of jealousy and a sense of not belonging in Hermione's new social circle.