It could be due to a misunderstanding about their respective roles in their group or in a mission. For example, if Sirius thought James was taking too much of a leadership role without consulting him, it might cause a rift. In fanfiction, the writer might explore how their strong personalities clash in such situations.
Perhaps a love interest is involved. If they both develop feelings for the same person, it could create a huge conflict between them. Fanfiction often delves into the complex emotions that come with such a situation, like how they might initially deny their feelings and then lash out at each other.
They could have an honest conversation. Just sit down and talk about what's been bothering them. In fanfiction, this could be a very emotional scene where they both realize they've been acting silly.
Jealousy could also play a part. For example, if Harry starts getting closer to other mentors or friends, Sirius might feel left out and act out in a way that causes a fight. Or if Sirius seems to be spending more time on Order of Phoenix business than with Harry, Harry could feel neglected and start an argument.
Another reason might be the stress of their quests. They are constantly under pressure to save the world or their friends. This stress can build up and cause them to lash out at each other. For instance, if a quest isn't going well and they keep hitting dead ends, they might start blaming each other for mistakes made along the way.
In fanfiction, Sirius and Remus could fight over Hermione because of their different relationships with Harry. Since Hermione is Harry's close friend, Sirius might think that he should have more influence over Hermione to help Harry better, as he is Harry's godfather. On the other hand, Remus, being Harry's former teacher and friend, also feels a sense of responsibility towards Hermione because of her connection to Harry. This clash of responsibilities and their own personal feelings for Hermione can spark a fight.
One reason could be jealousy. Snape might be jealous of James' popularity and charm. James, on the other hand, could be jealous of Snape's close friendship with Lily in their early years. Another reason is their different personalities. James was outgoing and bold, while Snape was more introverted and brooding. Lily might be attracted to different aspects of each of them, which could lead to the fight as they both compete for her attention.
Their own personal histories could also be a factor. Remus has always been an outsider in some ways because of his werewolf nature, and he might see Hermione as someone who accepts him completely. Sirius, on the other hand, has had a troubled past and Hermione's loyalty and goodness might be very appealing to him. So, they could fight over who 'deserves' her more based on their own self - perceived worth. And also, they might be fighting not just for Hermione but for a sense of normalcy and a future that she represents in their lives.
One reason could be that they might have different interpretations of Harry's character in the fanfiction. Hermione, being logical, may see Harry in a more practical light, while Ron, with his emotional side, could have a different view.
If he failed to follow the rules of the Marauder's Map. Maybe he used it in a way that endangered himself or others. His family, who are well - aware of the power and potential misuse of the Map, would ground him to ensure he understands the importance of using magical items responsibly. In his grounded state, he could be exploring the history of the Map and learning from the mistakes of previous users.
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.
In some fanfictions, their fight might be due to a competition at Hogwarts. Maybe they are both vying for the top spot in a particular class or for a special award. Hermione's studious and competitive nature combined with Draco's pride and desire to outdo others could easily result in a fight.