One possible reason could be a misunderstanding. Maybe Spider - Man is framed for a crime he didn't commit, and the Avengers, being the protectors of the world, believe the false evidence against him. For example, a powerful villain could have planted evidence to make it seem like Spider - Man was involved in a large - scale attack on a city. Another reason could be a difference in ideology. If Spider - Man starts to take actions that the Avengers think are too extreme or irresponsible in his fight against crime in New York, they might decide to hunt him down to stop him from causing more potential harm.
In some fanfictions, it could be due to a power imbalance. Spider - Man might gain an extremely powerful new ability that the Avengers fear could get out of control. They may think that hunting him is the only way to contain this potential threat to the world.
Perhaps there's a mind - control element involved. A new enemy with mind - control powers could manipulate Spider - Man into doing things that appear to be threats to the Avengers. As a result, the Avengers see him as a danger and start to hunt him. Or it could be related to a secret that Spider - Man discovers. If this secret is something that the Avengers don't want to be revealed, they might try to stop him by hunting him down.
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.
Spider - Man would likely be confused at first. He respects the Avengers and can't understand why they are suddenly after him. He might try to talk to them, to explain that he's innocent. For example, he could swing up to Captain America and start pleading his case.
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.
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.
It could be due to a mind - control device. Some evil force might be controlling the Avengers and forcing them to torture Spider - Man. This would add an element of helplessness to the story as the Avengers are not acting of their own free will. Another reason might be a deep - seated jealousy within the Avengers. Perhaps they feel that Spider - Man is getting too much attention and they want to put him in his place, which is a very dark and out - of - character take but possible in the realm of fanfiction.
One possible reason could be a misunderstanding. Maybe the Avengers think Spider - Man has been compromised by some sort of enemy influence. For example, if a powerful villain has the ability to control minds and they suspect Spider - Man is under that control, they would try to capture him to protect the world and also to try and break the control. Another reason might be that they believe Spider - Man has some information that is crucial for saving the world but he's being uncooperative, so they resort to capturing him. It could also be that there's some sort of new law or regulation in the superhero world that Spider - Man is unknowingly breaking, and the Avengers, as the enforcers of a sort of superhero code, feel they need to bring him in.
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.
In a fanfiction scenario where the Avengers kidnap Spider - Man, it could be because of a prophecy. The Avengers might believe that Spider - Man will cause great harm in the future according to some ancient prophecy. So, they take him against his will to a secret location. However, Spider - Man manages to convince some of the younger Avengers, like Peter Parker's friends among them, that he's not a threat.
Well, it could start with Spider - Man being framed for a crime he didn't commit. The Avengers believe in his innocence. They fight for him against a corrupt government agency that wants to arrest him. Along the way, they discover that there's a bigger conspiracy at play. Black Widow uses her espionage skills to gather intel, Hulk provides the brute force when they face the agency's armed forces, and the other Avengers contribute in their own ways to clear Spider - Man's name and protect him from harm.
Well, one possibility could be that Spider - Man is mind - controlled and ends up attacking the Avengers. Maybe a villain uses some sort of advanced technology or magic to make him do it. He might be forced to use his web - slinging and super - strength against his allies, causing harm both physically and emotionally to the Avengers and their fans who look up to their unity.