One possible reason could be Edward's overprotectiveness. He might restrict Bella's actions too much in the fanfiction, which could lead to her feeling suffocated and result in a fight. For example, if he doesn't let her go out with her friends or participate in certain normal human activities.
Another reason could be a misunderstanding. There could be some false rumors or misinterpreted actions. For instance, if Bella is seen with another male vampire by Edward's family and they misreport it to Edward, he could get angry and start a fight with Bella without knowing the truth.
One reason could be Edward's overprotectiveness. He might try to control Bella's actions too much in the fanfiction, which could lead to her feeling suffocated and result in a fight. For example, if he stops her from seeing her human friends often.
One reason could be a misunderstanding about their family's future. Maybe Edward has a vision of how the family should evolve that Carlisle disagrees with. For example, Edward might want to expand the family in a different way than Carlisle.
It could also be due to misunderstandings. Fanfiction often plays with the idea of miscommunication between them. Maybe Bella misinterprets Edward's over - protectiveness as a lack of trust in her ability to handle vampire - related situations. Or Edward might misunderstand Bella's need for independence as a sign that she doesn't love him as much as he thought. These misunderstandings can escalate into arguments.
One possible reason could be that Edward feels he is putting Bella in too much danger. In the 'Twilight' universe, his vampire nature attracts various threats. Another reason might be that he realizes their different life expectancies and doesn't want Bella to have a future filled with sorrow. Maybe he also fears that his control over his vampire instincts might fail one day and he could harm her.
One possible reason could be that Bella realizes she wants a more normal life. In the Twilight series, Edward's vampire nature brings a lot of danger and secrecy. Maybe in the fanfiction, Bella gets tired of constantly being in the middle of vampire - related dramas and decides she can't handle it anymore.
One possible reason could be that Paul represents a different kind of life for Bella. Maybe he is more down - to - earth compared to the often brooding Edward. Paul might offer a simpler and more straightforward relationship without all the supernatural drama that comes with Edward.
One possible reason could be that Bella discovers a different side of Jasper that she finds more appealing. Maybe Jasper shows a deeper understanding of her in some aspects that Edward lacks. For example, Jasper's past experiences might make him more empathetic towards Bella's own struggles in a way that Edward, with his own set of vampire characteristics, doesn't quite get.
One possible reason could be that Bella discovers a side of Rosalie that she feels more connected to on an emotional level. Maybe Rosalie shows her a different perspective on the vampire world that Edward had been hiding from her. Another aspect could be that Bella has grown tired of Edward's over - protective nature and sees Rosalie as a more independent and free - spirited vampire to be around.
Perhaps in the fanfiction, Edward broke a significant promise to Bella. Promises are important in relationships, and if he failed to keep one, it could lead to her not forgiving him. For example, if he promised to always be there for her but then abandoned her in a time of need. Also, his over - protective nature in a wrong way could make Bella feel suffocated and less likely to forgive him.
One possible reason could be that he suddenly realizes he's putting Bella in too much danger. In the Twilight universe, Bella's life is constantly at risk because of Edward's vampire world. Maybe in the fanfiction, he decides at the last minute that he can't bear to see her hurt because of him.