In fanfic, sometimes the writers want to create drama. By making the Avengers assholes to Tony, they can build up conflict. It could also be related to past events that the fanfic author has created. Maybe Tony made a mistake in the past that the other Avengers can't forgive, even though it was an accident or done with good intentions, and they keep treating him badly.
Another possible reason is that the Avengers in the fanfic have different moral compasses. Tony's actions might not always align with what the others consider to be right. For example, his use of weapons technology in the past could make some of the Avengers look down on him, and they express this through assholish behavior.
Differences in ideology can also play a part. Tony might have some views on how to handle threats or use technology that the other Avengers don't agree with. Maybe he's more willing to take risks with untested tech that could endanger the team or civilians, which would lead to resentment.
It could also be about narrative tension. Fanfic authors want to keep the story interesting. By having the Avengers be 'dicks' to Tony, they create a source of conflict. This conflict can drive the story forward, whether it's leading to Tony leaving the team for a while, or him trying to fix the relationships. It gives the author more room to explore different emotions and plot developments, like Tony's journey to regain the trust and respect of his fellow Avengers.
One possible reason is that Peter's quick rise to popularity among the public might make the Avengers jealous. They've been around for a long time, and suddenly this young kid is getting all the attention.
One possible reason could be a misunderstanding. Maybe Peter accidentally gets involved in something that looks bad to the Avengers, like being in the wrong place at the wrong time during a security breach. So they misjudge him and are mean as a result.
He might dislike it because it can create false expectations. Fans read fanfiction and might start expecting the real Avengers to act in a certain way that is not in line with their true nature.
In some fanfictions, Peter could have a secret that he's keeping from the Avengers. When it's eventually revealed, they might feel betrayed and turn against him. It could be something related to his powers, or his past associations that the Avengers find unacceptable, thus resulting in their hatred.
One reason could be to explore the extreme consequences of their ideological differences. In the official story, they had disputes over things like the Sokovia Accords. In fanfiction, this could be pushed to the limit, leading to Tony taking Steve's life.
One reason could be to explore the darker side of the characters. Some fans like to see how the Avengers could be pushed to their limits and do something out of character, like hurting Tony as a teenager. Another reason might be to create a more angsty and emotional story, with Tony as a victim and the Avengers as the antagonists, which can add a lot of drama.
One possible reason could be Steve's strong sense of loyalty to Bucky. Maybe he feels that he owes Bucky so much after all they've been through. He might be so focused on helping Bucky or being there for him that he unconsciously ignores Tony.
One possible reason could be jealousy. Maybe Tony's intelligence and wealth made some Avengers feel inferior, and over time, that festered into betrayal. For example, if Tony developed a new technology that could potentially overshadow their own abilities, they might feel threatened.