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.
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.
One possible reason could be that he's gotten on the wrong side of a powerful client. Maybe he uncovered some shady business deals of the client during a case, and the client decided to take revenge by having him beaten up. Another reason might be related to his past. If some of his old associates from his pre - Harvey days found out he was now working in a high - profile law firm, they could be jealous and target him.
In a fanfiction about Tanjiro getting sick, it could be that he overexerts himself during a mission. His body gives in and he falls ill. His comrades take him to a safe house. While he is sick, he reflects on his journey so far, his losses and his goals. Meanwhile, the others keep a lookout for any danger and also try to nurse him back to health. They might face shortages of supplies and have to make difficult decisions.
He might be in shock at first. Mike could just sit there, trying to process what has happened. Then, he would probably be angry. He's not the type to just let things go, so he'd start thinking about who could be behind it and how to get revenge.
Lucy getting beat up can also be a turning point for her character development. It might make her realize she needs to train harder or develop new magic skills. This could lead to her going on a training journey, which would be an interesting sub - plot in the fanfiction.
One possible reason could be that she's in the wrong place at the wrong time. Maybe she stumbles upon a group of antagonists who target her because she's known for her association with certain powerful characters. For example, if there are enemies of her friends or family, they might use her as a way to send a message.