One reason could be their different outlooks on the Cullen family's future. Edward may have ideas that Rosalie disagrees with, like how involved they should be with the outside world or other vampire clans. Rosalie might be more concerned about protecting the family's status quo and secrecy, and Edward's actions could seem to her as jeopardizing that.
The fight could be because of their individual relationships within the family. Edward has a special bond with Carlisle, and Rosalie may feel left out or that her own relationship with Carlisle is being overshadowed. This could lead to resentment and ultimately a fight between her and Edward.
One possible reason could be their rivalry over a love interest, like Bella. Jacob loves Bella and Edward is in a relationship with her. This love triangle often leads to tension and could easily erupt into a fight.
In fanfiction, Edward and Rosalie might fight due to their different personalities. Edward is often more brooding and reserved, while Rosalie can be quite headstrong. Their fight could be about protecting Bella. Maybe Rosalie thinks Edward is putting Bella in too much danger with his vampire world, and she wants to keep her away from it for Bella's own good.
Well, in many fanfictions, Rosalie might be jealous of Bella. Rosalie has always had a complex relationship with her vampiric existence and seeing Bella so easily adapt to it could stir envy. Also, Rosalie is very protective of the Cullen family, and Bella's presence could be seen as a threat to the family's secrecy or stability.
One possible reason could be Renesmee's growing independence. As she matures, she might want to make her own decisions which could conflict with Edward's over - protective nature. For example, she may want to explore the world more freely, while Edward, fearing for her safety, tries to restrict her.
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.
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.
A possible plot might involve Rosalie having some sort of change in her feelings towards Edward. She might start to show him more affection than usual. Edward, being a bit confused by his own emotions, succumbs to her advances while still being in a relationship with Bella. This could lead to a lot of drama, with Bella finding out and feeling heartbroken.
Perhaps it's related to his internal conflict regarding his relationship with Bella. He might be feeling so much guilt about bringing her into the dangerous vampire world that he has a moment of self - destructive behavior where he almost kills her. Or it could be a plot device used by the author to create more drama and tension in the story, making the readers more engaged as they wonder how Bella will survive and how their relationship will recover from such a traumatic event.
The allure lies in the fact that we can see different sides of Rosalie and Edward. In fanfiction, we might see a more vulnerable Edward or a more forgiving Rosalie. It gives fans the chance to rewrite their relationship in a way that satisfies their own desires for the characters.
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.