There could be a rift in the group he belongs to. If it's within a group like the Gryffindor common room or among his close friends, the punching incident could cause sides to be taken. Some might side with the person who punched Ron, thinking he deserved it for his betrayal, while others might feel that punching was too extreme, creating tension and division within the group.
He could become more isolated. After being punched, his friends might avoid him for a while. For example, if Harry punches him, Harry might need some time to cool off and not want to be around Ron. This would make Ron feel really lonely and regret his actions that led to the betrayal.
Well, it could be Harry who punches Ron. Maybe Ron betrays Harry's trust in some way, like perhaps revealing a secret of Harry's to the wrong people. And Harry, being really angry about it, punches him in a moment of rage.
It could also lead to a divide within their group of friends. Hermione, for instance, might be caught in the middle, not knowing who to side with. This could create a lot of tension and conflict among the characters, adding more drama to the fanfic. If the punch was public, it could also damage Ron's reputation among their peers, as punching a friend is not an acceptable behavior in most cases.
It could be a misunderstanding. For example, Ron might think Harry did something really bad to Hermione or their other friends. Without proper communication, his anger could lead him to punch Harry. Also, if there was some sort of magical influence or dark magic at play that affected Ron's judgment, that could be a reason too.
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.
I'm not sure about a specific 'ron punched harry fanfic' off the top of my head. You could try searching on fanfic websites like Archive of Our Own. They have a large collection of Harry Potter fanfictions and you might be able to find one with that plot there.
Maybe it's a story where Hermione accidentally gets caught in the middle of a wizard duel gone wrong. One of the angry duelists punches her in a fit of rage. Then the story could focus on how she deals with the injury and the chaos that follows, as well as her pursuit of justice for the unprovoked attack.
One possible scenario could be that it's a case of mistaken identity. Maybe someone thought Annabeth had done something wrong when she hadn't. Another could be in the heat of an argument where emotions got out of control and someone lashed out at her. Or perhaps it's part of a more complex plot where an enemy is trying to send a message to her friends by punching her.
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.
The cartoon dog getting punched might be for creating drama or adding an unexpected twist. Maybe it's a way to introduce a lesson or moral to the viewers.
Maybe he did something wrong to deserve it.