He could be exploring an old, abandoned building and fall through a rotten floorboard.
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.
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.
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 possible fanfiction could be where they are in a battle against a powerful new villain. Peter Parker might get hurt while trying to protect civilians, and Tony Stark, in his attempt to save Peter, also gets injured. This could lead to a heart - wrenching scene where they both have to rely on each other's strength to get through the situation and back to safety.
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.
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.
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.
One reason could be his own clumsiness. Maybe Peter is always in a rush and not paying enough attention to his surroundings. For example, he could be running down the stairs while looking at his phone and trip, resulting in an injury.
Another possibility is that the fanfiction writer wants to explore a different side of their relationships. By having Hermione and Ron hurt Harry, it shows the complexity of human (or in this case, wizard) emotions. It could be a way to show that even the strongest friendships can have cracks and that mistakes can be made. It also allows for character development as Harry has to deal with this new situation, and Hermione and Ron may later realize their mistakes and try to make amends.
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.