One possible reason could be that he faces repeated betrayals in the fanfiction world. Maybe his friends turn against him, which shakes his belief in the goodness of people around him.
If someone were to insult or harm Hinata, who he loves deeply, Naruto could lose his temper. He's very protective of those close to him, and any threat to Hinata would likely trigger his anger. He has a strong sense of justice and loyalty, so seeing someone he cares about being mistreated would be a big deal.
One possible reason could be the loss of his friends. Naruto deeply values his relationships, and if something were to happen to his friends like Sasuke leaving again or getting seriously injured, it would shatter his happiness. Another reason might be if he fails to protect the village. Naruto has always seen himself as the protector of Konoha, so if he fails in this duty, his sense of purpose and happiness would be lost.
Another reason might be related to a plot device in the fanfiction. The author could be using the loss of the mask to create a turning point in the story. It might lead to new relationships being formed or old ones being tested. For instance, his friends might react in different ways when they see his face without the mask, and this could drive the story in different directions.
Maybe he was in a dangerous duel with a powerful dark wizard. The dark wizard could have cast a spell that was specifically designed to maim or disarm in the most literal sense.
One reason could be that some fans are very purist about the original Naruto story. They might feel that fanfiction often distorts the characters or the plot in ways that go against the essence of what Naruto is all about. For example, changing a character's personality completely to fit a romantic or dark and edgy narrative that doesn't align with the source material.
There are several possible reasons. For one, it could be a way to subvert the expectations of the Naruto character. In the original series, he's so pure and full of life. By having him smoke in fanfiction, writers can create a contrast. It might also be used to set a certain mood, like a gritty or more adult - oriented atmosphere. Additionally, it could be a symbol of his rebellion or a sign of him breaking away from the norms of his ninja village. Writers might use it to explore a different side of Naruto that wasn't shown in the original story.
One reason could be the waning popularity of the Naruto franchise over time. As new shows and series emerge, some fans shift their attention away. Another factor might be that the quality of fanfiction has declined. With so many people writing, there's a lot of poorly written stuff that turns readers off.
One reason could be to explore a different character arc. Writers might want to see how Naruto would develop if he had a more violent nature from the start. Another reason is to add drama. A bloodthirsty Naruto would create more conflict within the story, especially with his friends and the village. Also, it could be a way to show a more 'realistic' or gritty version of the Naruto world, where characters are not always so morally upright.
One reason is that some fanfiction authors may lack a deep understanding of Naruto's character. They might write things that are completely contrary to his values and personality just for the sake of creating a 'unique' story. For example, Naruto is known for his positive attitude and strong will, but in some fanfictions, he could be depicted as a negative or spineless character.
Maybe it was due to a fierce battle. The enemy could have targeted the equipment related to his wings, like shooting down the vehicle or device that supported the wings' function.