One possibility is that there's a misunderstanding between Naruto and Karui. Maybe Karui thinks Naruto has something to do with Sasuke's actions that have affected her village. So, in her anger, she attacks Naruto.
Sasuke could be one. Given their complex relationship, there could be a situation where Sasuke's newfound power or different ideology leads him to fight and beat Naruto in a fan - made story.
In some fanfictions, Naruto might get beat up when facing a powerful new villain who has some unknown jutsu. This could be a villain created by the fanfic writer to test Naruto's limits.
Naruto might be shocked at first. He has always seen Sakura as a friend, so being beaten up by her would be unexpected. He could try to talk to her and figure out what's wrong.
Well, in a lot of these fanfictions, Stiles can get beat up due to his own recklessness. He might be so determined to solve a mystery or help his friends that he doesn't think about the consequences and ends up in a dangerous situation where he is outnumbered and beaten up. Also, there could be miscommunication. Maybe someone thinks he has done something wrong when he actually hasn't, and they take matters into their own hands and beat him up.
Maybe some new kids in the neighborhood who are trying to establish their dominance and pick on Lincoln as an easy target. They might not know much about him or his family, but just see him as someone they can bully.
It could be an antagonist character in the Septiplier fanfiction world. Maybe someone who has a grudge against Jack for some reason, like a rival in a game or a love interest situation gone sour.
Their relationship could develop through shared experiences. For example, if they both participate in a big battle against a common enemy. They protect each other during the fight, which makes them see each other in a new light.
Well, in a 'jack gets beat up septiplier fanfiction', it could be that the writer is exploring a darker side of the relationship dynamics within the Septiplier fandom. It could be that Jack has made an enemy within the story's context, perhaps someone jealous of his relationship with another character. The beat - up could be a way to create drama and tension in the fan - created narrative.