In some fanfics, the Avengers capture Spider - Man because they want to study his powers. His unique abilities, like his spider - sense and wall - climbing, might be of great interest to the Avengers' scientific minds, so they take him in to run some tests.
A common reason in fanfics is that the Avengers could be misled by a powerful villain who has framed Spider - Man. The villain might have planted false evidence or used mind - control on people around Spider - Man to make it seem like he's turned bad. So, the Avengers, believing the false information, 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.
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 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 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.
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.
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.
One interesting storyline could be Spider - Man teaming up with the Avengers to fight a new super - villain that's a combination of different powers they've faced before. For example, a villain with the strength of the Hulk and the mind - control abilities of Loki.
One interesting Spider - Man Avengers fanfic could be 'Spider - Man's New Allies in the Avengers'. It might show how Spider - Man adjusts to being part of the big superhero team. Another could be 'Spider - Man Saves the Day for the Avengers' where he uses his unique skills to get the Avengers out of a tough situation. There are also fanfics that focus on his relationships with individual Avengers, like his mentorship under Iron Man.