Another possibility is that in the fanfic's unique plot, there could be some sort of magical influence or dark magic at play. Perhaps a spell was cast on Ron that made him act out of character and punch Harry. It could also be part of a complex plot where Ron is being controlled by an outside force to create drama between him and Harry in the context of the Drarry relationship.
In some fanfictions, Ginny might be mad at Harry because of a misunderstanding. For example, she could misinterpret his actions towards another girl as being unfaithful. Maybe he spends a lot of time with Hermione on a project and Ginny thinks there's something more between them.
Maybe Ron is jealous of Harry and wants to have something that belongs to him for a change. The cloak gives Harry a certain freedom and mystery, and Ron might be longing for that kind of experience.
It could be that Ron's actions were causing problems within the Order of the Phoenix. If Ron was being stubborn and not following the plans or orders set by the Order, which Sirius is a part of, Sirius might get angry. He would be frustrated with Ron for not being a team player and potentially endangering the mission and the lives of others.
There could be a 'Harry punches Ron' fanfic to explore different character interpretations. Maybe the writer wants to show a more impulsive and hot - headed Harry, or a Ron who has done something really wrong that would push Harry to such an extreme. It can also be a way to break away from the typical, harmonious view of their relationship and see how they would deal with such a violent rupture in their friendship.
There are several possible reasons. It could be due to a misunderstanding about a prophecy or a magical object. For instance, if Ron accidentally breaks a magical heirloom that Harry believes could be important in fighting Voldemort, Harry might be so angry that he ignores Ron. Also, it could be related to Ron's family. If Ron's family does something that Harry disapproves of, like siding with the Ministry when Harry thinks they're wrong, Harry could start ignoring Ron.
In some fanfictions, Harry might be angry with Ron and Hermione if he feels they are keeping secrets from him. For example, if they discover something important about Voldemort's plans but don't share it right away, Harry could get angry. He's used to relying on them and when that trust is broken, even if it's a misunderstanding, his anger can flare up.
Maybe he loves the idea of the Firebolt in the fanfiction. It could be that the stories expand on his experiences with the Firebolt in a really exciting way.
Perhaps in the fanfiction, Sirius is mad at Harry because Harry has befriended someone Sirius doesn't trust. Sirius, being protective of Harry, might see this new friendship as a threat. He could be worried that this person will lead Harry astray or put him in danger, and his anger towards Harry stems from his concern for Harry's well - being.