There's a possibility that it's an old enemy who has somehow regained power or found a new way to attack. For example, Frieza's family or the Red Ribbon Army if they had some kind of resurgence. They might target Vegeta and Bulma out of revenge or to gain access to Capsule Corp's technology which Bulma controls.
In some fanfictions, it could be an accident or the result of their own actions. Maybe Vegeta was training too hard and lost control, accidentally endangering Bulma. Or Bulma's experiments in Capsule Corp could go awry and cause harm to both of them without any external malicious intent.
Another possibility is a jealous student at Hogwarts. There are always students who might be envious of Hermione's academic achievements. They could set up a prank or a trap that goes wrong and ends up hurting her. For example, a student who is tired of Hermione always getting the top grades in Potions might tamper with her cauldron during a class experiment, resulting in an explosion that hurts her.
As I mentioned before, a member of his own wolf pack. In a pack, there can be jealousy and power struggles. If another wolf wants more influence or has a grudge against Jacob, they might be the ones to cause him harm.
It could be an enemy that manages to sneak onto the Argo 2. Maybe a monster that they thought they had defeated before but actually just bided its time and then attacked Percy when he was least expecting it.
It could be a rival from another house. For example, a jealous Slytherin who wants to take Ginny out of the Quidditch game. They might use some underhanded tactics to make her broom malfunction and cause her to get hurt.
Maybe it's a jealous acquaintance. If Kagami is really good at something, like she's a star in a particular activity, someone who is jealous of her success might set up a situation where she gets hurt. For instance, they could tamper with her equipment in a way that causes her to have an accident.
In some cases, it could be herself. Maybe she pushed herself too hard in her work or training, and her body or mind couldn't take it anymore. This self - inflicted harm could be a key part of the fanfiction's plot, as she then has to deal with the consequences and learn from her over - zealousness.
It could be an enemy from the underworld. Maybe a powerful demon or a vengeful spirit that has a grudge against her. They might use their dark powers to harm her, like sending a bolt of dark energy her way.
It could be an over - enthusiastic fan at an iCarly event. The fan might be so excited to meet Carly that they accidentally knock her down while rushing towards her.
His siblings could be responsible. They are always getting into crazy situations together, and maybe one of their pranks or games goes too far and accidentally hurts Lincoln.
It could be another animal in the neighborhood. Maybe there's a territorial cat that doesn't like Jojo and Saphie being around. This cat could attack Jojo when they cross into its territory.