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.
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.
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.
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.
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.
One possible reason could be a misunderstanding. Maybe some villains have framed Spider - Man in a way that makes the Avengers believe he's a threat. For example, Mysterio could have created false evidence to make it seem like Spider - Man is involved in some evil plot against the world.
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 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 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.
Well, maybe he's got some secret mission that he doesn't want the Avengers to interfere with. You know, like protecting a particular person or some crucial information that he thinks the Avengers might mess up if they get involved.
Spider - Man would probably be shocked at first. He looks up to the Avengers and being kidnapped by them would be the last thing he expects. He'd likely try to talk his way out, using his quick wit and charm. He might start by asking why they're doing this and try to convince them that they've made a mistake.