Another reason could be her relationships. If she has a fight with a friend or a love interest. Let's say she and Edward had an argument over his over - protectiveness. That could easily make her lose her cool.
Perhaps she is frustrated with her own limitations. If she feels like she can't do something that others can, like being a normal human in a world full of supernatural beings. This sense of inadequacy might lead to her losing her temper.
It could also be due to a misunderstanding. Rosalie might say or do something that Bella misinterprets. Say, Rosalie gives some advice about Bella's relationship with Edward, but Bella takes it the wrong way, thinking Rosalie is trying to interfere in a negative way and thus loses her temper.
Rosalie might initially be taken aback. Since she is used to being in control and having a certain level of respect, Bella's outburst could shock her. She might just stand there for a moment, not knowing how to respond immediately.
If someone were to insult or harm Hinata, who he loves deeply, Naruto could lose his temper. He's very protective of those close to him, and any threat to Hinata would likely trigger his anger. He has a strong sense of justice and loyalty, so seeing someone he cares about being mistreated would be a big deal.
One reason could be for a disguise. Maybe she's on the run from some bad guys in the fanfiction world and changing her hair color helps her blend in better.
One possible reason could be that she discovers some dark secrets about the Cullens. Maybe she finds out they have been lying to her about their true nature in a more malicious way than in the original story. Another reason might be that she feels threatened by their vampire powers. If they accidentally or purposefully use their powers around her in a menacing way, it could make her feel scared and turn that fear into hatred.
One possible reason could be a misunderstanding. Maybe Harry overhears Ron and Hermione having a private conversation where they seem to be critical of him in some way, but they were actually just worried. Another reason might be if Harry starts to pursue a very different path in magic, like dark magic perhaps, and Ron and Hermione can't support or be associated with that. It could also be that a new character comes into their lives who tries to drive a wedge between them by spreading false rumors about each of them to the others.
One possible reason could be a vampire attack gone wrong. Maybe a rogue vampire targeted Bella and inflicted a near - fatal wound. Another reason might be related to some sort of dark magic or a curse that was unknowingly placed on her. It could also be due to a self - sacrifice situation where she put herself in harm's way to save someone she loves, like Edward or one of her friends.
One possible reason could be that he's constantly being underestimated or not given the credit he deserves. In some fanfictions, his friends might take his contributions for granted, which could lead to his temper flaring up.
Maybe Bella has a warped sense of what's good for her sister. She could think that the vampire fanfiction world offers a form of escapism or a different perspective on life that she thinks her sister needs. It could also be related to a relationship she has with a character in the fanfiction. If there's a vampire character she admires or is in love with, she might want her sister to be part of that world so that she can better understand Bella's feelings towards that character.
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.