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.
One possible reason could be to rescue a friend or ally who is in trouble there. Maybe someone he knew from his previous time in Afghanistan got caught up in a new conflict or was kidnapped by a local militia.
He might first feel a sense of despair and loss. But then he could start to rely on his other skills and intelligence. For instance, he could focus more on his strategic planning and use his knowledge of technology to help his team in other ways.
Another possibility is that he underestimates the importance of family. Tony has always been focused on his own achievements and the big - picture stuff. He might not realize how much his daughter needs him until it's too late.
Maybe he's too caught up in his work. Tony Stark is often depicted as a workaholic, so he could be so focused on his inventions and saving the world that he simply doesn't have time for his son.
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.
It could also be due to external pressure. The media or some powerful figures in the fanfiction world might force Tony to stay away from Peter. They could spread false rumors about their relationship or use their influence to separate them for their own gain.
One possible plotline could be that Tony Stark discovers a new technology that allows him to have wings. He could use these wings to enhance his superhero capabilities, like flying faster and more stealthily than with his regular Iron Man suit. Maybe he uses the wings to access hard - to - reach places during a rescue mission or to surprise his enemies from above.
One reason could be a misunderstanding. Maybe Kate did something that Tony misinterpreted, leading to him telling her off. For example, if she made a decision without consulting him in a shared project within the story's universe.