A common reason is battles. Sasuke is often involved in intense combat situations in fanfiction, whether it's against old foes with new powers or entirely new enemies. His determination to prove himself or protect something often leads him to take risks that result in getting hurt.
Sometimes, it's due to betrayal. If he's betrayed by someone he thought was an ally in the fanfiction, they might attack him when he least expects it, causing him to get hurt. This adds an element of drama and can lead to interesting developments in the story as he has to deal with the physical injury as well as the emotional wound of betrayal.
One common reason could be Sasuke's thirst for revenge. His single - minded focus on getting stronger and avenging his clan might make him overlook Sakura's feelings and accidentally hurt her. For example, he might leave her behind in a dangerous situation while chasing after an enemy related to his clan's massacre.
Equipment failure can be a factor. In fanfiction, his web - shooters could malfunction. Maybe they run out of web fluid at a crucial moment during a fight, leaving him without his main means of movement and defense. As a result, he's more exposed to attacks from villains and gets hurt.
Another reason could be external threats that target Starfire specifically while Robin is trying to protect her but fails in some way. For example, a powerful villain might know of Starfire's weaknesses and use them against her, and Robin's attempts to counter the villain's attacks might not be enough in time, thus she gets hurt.
The environment in which the story takes place can also be a reason. For instance, if the fanfiction is set in a post - apocalyptic Gotham, Batman has to deal with harsher conditions, more desperate criminals, and less resources. All of these can contribute to him getting hurt more easily as he tries to maintain order and fight crime.
He could be exploring an old, abandoned building and fall through a rotten floorboard.
One possible reason could be a misunderstanding during a mission. Maybe Erza's actions were misinterpreted by Natsu, leading to him getting hurt, either physically or emotionally.
One common reason is his inexperience. He's still a young superhero, and in the chaos of the Civil War situation, his lack of experience can lead to mistakes. Another reason could be his loyalty. He might put himself in harm's way to protect someone he cares about, like Tony or Steve, without fully considering the risks.
One reason could be that he was being a bit too adventurous. He might have tried to climb a tree that was too high or unstable for him. As he reached for a higher branch, he lost his balance and fell, getting hurt in the process. Also, bad weather conditions can play a role. If it had rained recently and the ground was slippery, Chris could have slipped and fallen while walking in the park.
One reason could be a jutsu gone wrong. Maybe an enemy uses a forbidden jutsu on Sasuke that has an unexpected side effect of changing his gender.
In some fanfictions, Sasuke might get hurt by a curse that was placed on him long ago. As he suffers, he has visions of his past, his family, and his mistakes. His struggle with the physical pain and the emotional turmoil of his past makes for a very engaging and complex storyline.