One possible reason could be a difference in ideology. Maybe Spider - Man has a more local - friendly approach to crime - fighting while the Avengers are often involved in large - scale, global threats. He might feel that his skills are better utilized in his own neighborhood, like Queens, rather than in the grand, often cosmos - level battles of the Avengers.
It could also be due to personal reasons. For example, he might be worried about endangering his loved ones. If being an Avenger attracts more powerful and dangerous enemies to him, he may choose to leave to protect Aunt May, MJ, and his friends. In the fanfiction world, his personal relationships often play a big role in his decisions.
One reason could be that he feels ostracized. Despite his efforts to protect Konoha, some villagers still look at him with suspicion because of the Nine - Tails inside him. He might just get tired of constantly having to prove himself and decide to leave in search of a place where he can be accepted for who he is.
Another possibility is that the Avengers are being manipulated by a villain. The bad guy could be using false information to make the Avengers believe that Spider - Man is a danger. So they go after him without really knowing the truth.
One possible reason could be a difference in ideology. Maybe Peter Parker feels that Stark's methods are too extreme or not in line with his own moral compass. For example, Stark might be involved in some projects that Peter doesn't agree with, such as using certain unethical technologies in the pursuit of justice.
One reason could be a misunderstanding. Maybe some events were misinterpreted, and the Avengers thought Spider - Man was involved in something bad. For example, if there were some crimes that seemed to have Spider - Man's fingerprints all over them, but in reality, he was being framed.
One possible reason could be a misunderstanding with his friends in Gryffindor. Maybe there was a big argument over something like a Quidditch match strategy that made him feel alienated.
In the fanfiction, it could be that Spider Man has a personal vendetta against one of the Avengers members. For example, if one of them had a past with a villain that Spider Man has a strong connection to, like if an Avenger unknowingly helped the villain in the past, Spider Man might transfer his hatred for that villain to the Avenger and by extension, the whole team.
One possible reason could be to escape the danger in England. Given the dark forces that were at play, especially Voldemort's rise, they might have felt it was safer to leave. Another reason could be for a fresh start. Maybe they wanted to explore new magical cultures and communities in other parts of the world.
Perhaps she has a new mission or calling that takes her away from Camelot. It could be something related to the greater magical world. For example, there could be an ancient magic that is in danger of being lost or misused, and she is the only one who can save it. Since Camelot has been rather unkind to magic in general, she might feel that she has to leave to fulfill this important task.
One possible reason could be ideological differences. Maybe she discovers some practices within the Order that go against her moral compass. For example, if the Order starts to use unethical magic in their fight against the dark forces, Hermione, with her strong sense of right and wrong, might not be able to tolerate it and decide to leave.
The plot might involve Spider - Man getting into some trouble in New York and the three Avengers coming to his aid. It could be that a powerful alien artifact has been stolen and Spider - Man accidentally gets blamed for it. The Avengers then step in to help clear his name and find the real culprit while also protecting the city from the potential danger of the artifact.