Another possibility is that Spider - Man has some sort of power or knowledge that the Avengers need. They might think that capturing him is the only way to get that information. Say he has a unique way of detecting a new and dangerous threat, and the Avengers want to force him to share that method.
It could also be that an outside force is manipulating the Avengers into believing Spider - Man is a threat. Some villain might be using mind - control or false evidence to make the Avengers go after Spider - Man. This would add an interesting twist to the fanfiction.
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. 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.
Spider - Man could use his agility and web - slinging skills to escape. He could swing between buildings, making it difficult for the Avengers to follow him. His small size compared to some of the Avengers also gives him an advantage in hiding in tight spaces.
One possibility could be that the Avengers capture Spider - Man because they suspect he has been influenced by some sort of mind - control or dark magic. Maybe they think he's a threat to the city without realizing he's actually trying to help. Another could be that there's a misunderstanding in which Spider - Man's actions are misinterpreted as being against the Avengers' interests.
Spider - Man would be very confused. He would feel betrayed because he thought the Avengers were heroes he could trust. He could start by asking them why they are doing this. If they don't give him a satisfactory answer, he might try to escape in a sneaky way, using his spider - like abilities to find a weakness in their capture setup while still trying to reason with them.
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 reason could be that they suspect him of being involved with a new villain's scheme without having all the facts. So they act preemptively and capture him.
Spider - Man would probably be very confused at first. He looks up to the Avengers and would wonder why they are after him. He might try to talk to them and explain that there has been a mistake.
The Avengers might capture Spider - Man if they suspect him of being involved with a new, unknown threat. For example, if there are strange crimes happening in the city and Spider - Man is seen at the scene every time, they could assume he's part of it.
One possible fanfic could be where the Avengers capture Spider - Man because they misunderstand his actions. Maybe they think he's working for an enemy when he's actually on a secret mission to help them. Another could be a story where a villain tricks the Avengers into believing Spider - Man has gone rogue.
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.