A mysterious new enemy that has infiltrated the village could be the cause. This enemy might be on a mission to disrupt the balance in the ninja world and sees Sarada as an obstacle. They could attack her when she is alone or on a solo mission. Their motives could be related to some ancient grudge against her family or the village as a whole.
In some fanfictions, it could be an accident caused by a malfunctioning piece of ninja equipment. Maybe she is testing out a new weapon or tool, and something goes wrong. It could be a weapon with a hidden defect that no one was aware of, and when she uses it in a training exercise, it backfires and causes her injury.
It could also be a new, unknown enemy. Maybe there's a secret organization that emerges in the fanfiction that has a vendetta against those with electric powers like Michael. They could target him specifically and cause him harm.
There are also the possibility of some inter - dimensional beings. In the Ben 10 universe, there are many different dimensions. If a being from another dimension comes through with the intention of causing chaos, they could target Ben. They might have powers that are completely foreign to Ben and his allies, making it difficult for him to defend against them. And let's not forget about the possibility of some of Ben's old enemies teaming up. If they work together, they could come up with a plan to really hurt Ben, whether it's physically or by ruining his reputation among the people he protects.
There could be a new character in the fanfiction, like a bully who targets Lincoln just because he seems like an easy target and ends up hurting him.
It could be a new villain that has emerged. This villain might have some sort of vendetta against Steven specifically, perhaps because of his unique powers or his role in the gem world.
Perhaps it's a mysterious figure in the fanfiction world. This unknown character could have some ulterior motives against Jacob and find ways to hurt him, like setting traps for him.
Even some of the school faculty could be responsible in a sense. For instance, a strict teacher who doesn't understand Danny's situation might push him too hard during detention or some other punishment, which could lead to an accidental injury. Or a science teacher's experiment could go wrong and Danny could get caught in the crossfire.
A plot could involve Sarada getting hurt in a trap set by a villain. The villain might be targeting the new generation of ninja in the village. As she gets injured, it sets off a chain of events where her friends start to investigate the incident. Sarada, while hurt, could provide some crucial clues from what she saw before getting injured. This would not only be about her physical injury but also about her mental strength as she tries to contribute to the solution while in pain.
There are several potential causes. Perhaps the Beast's temper gets the best of him for a moment and he accidentally causes her harm, although he would be extremely regretful afterwards. Or, there could be a new, malevolent character in the fanfiction, like a dark spirit that lurks in the forest around the castle and attacks Belle when she ventures out. It could also be that she was trying to help the Beast with something related to his curse and something went wrong, resulting in her getting hurt.
A powerful and ancient Pokémon from a long - forgotten era could be the cause. This Pokémon might have a grudge against Arceus for some past event and has finally found an opportunity to strike and hurt it. For instance, if Arceus had banished this Pokémon in the past, it could now be seeking revenge.
Hawkmoth, of course. He's always looking for opportunities to akumatize people, and Ladybug would be a huge prize for him. If he could find the right moment when Ladybug is feeling vulnerable, like after a big defeat or a personal crisis, he could send an akuma her way.