It really depends on the writer's intention. Sometimes Bella wins if the story is about her growth and empowerment. For instance, if the fight is about her independence, she might stand her ground and make Edward see her point of view.
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.
Another plotline might be a fight due to a misunderstanding about Bella's human friends. Edward could be overly protective and think that her human friends are putting her in danger, while Bella believes he's being too controlling. So, they have a big argument about her freedom and relationships with humans.
In some fanfictions, their fight might be over Bella's decision to become a vampire. Edward might be against it initially, fearing for her soul or something like that. Bella, on the other hand, could be really determined and they end up in a heated argument where she accuses him of not respecting her choice and he tries to make her see his point of view.
One possible drive for their physical fight could be jealousy. If Bella is seen getting too friendly with another character, Edward might become jealous and act in a way that makes Bella angry enough to start a fight.
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.
It could strengthen their relationship in the long run. After a fight, they might understand each other better. For instance, if they fight about Edward's secrets, once they resolve it, they could become more open with each other.
To explore different aspects of their relationship. Fights can show the weaknesses and insecurities in their bond. For example, through a fight, we can see how much they really trust each other or how they handle conflict, which are important elements of any relationship. In fanfiction, it gives writers a chance to put their own spin on the characters' relationship dynamics.
It can either strengthen or weaken their relationship. If they resolve their issues through honest communication during or after the fight, their bond could become stronger. They might understand each other better.
In a fanfiction, if Bella and Edward had a physical fight, it could be due to a misunderstanding. Maybe Edward misinterprets Bella's actions regarding another vampire. Their fight might start with some angry words and then escalate to a physical struggle. However, their deep love for each other would likely make them stop before any real harm is done.