Perhaps Rosalie accidentally endangers someone Bella loves. In the world of the fanfiction, if Rosalie's actions put Edward or one of Bella's human friends in harm's way, even if it was unintentional, Bella's protective nature could cause her to lose her temper at Rosalie.
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.
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.
It might make her friends feel worried or scared. For instance, if she suddenly loses her temper in front of her human friends, they might be taken aback and wonder what's wrong with her.
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.
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 bullying. If Percy is constantly being picked on by other students, it might push him to lose his temper. Maybe they tease him about his appearance, his family, or his abilities. Another reason could be academic pressure. If he's struggling with his studies and feels like he can't keep up, it could lead to frustration and ultimately a loss of temper.
One possible scenario could be when Merlin is constantly underestimated in Camelot. People might make fun of his clumsy appearance or dismiss his ideas. After a series of such incidents, he loses his temper. For example, if Arthur repeatedly ignores Merlin's warnings about a coming danger and then blames Merlin when things go wrong, Merlin might snap.
Well, in Twilight fanfic, the idea of 'Rosalie Seduces Bella' might stem from the desire to break the traditional relationship molds. The original series has its set relationships, and fanfic writers may want to introduce a more complex and unexpected relationship. Also, Rosalie has her own charm and allure, and some writers may think that it would be interesting to see how Bella would react to that kind of approach from Rosalie. It could also be a way to add more drama and conflict to the story, which is often what makes fanfic engaging.
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 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.