It could also be due to a parallel universe situation. In this 'fanfiction' universe, the events that led to the Avengers knowing Steve simply didn't occur. For example, Steve might have been on a different mission or in a different part of the world during the events that originally brought them together.
One possible reason could be memory alteration. Maybe some villain has the power to manipulate memories and has erased Steve from the Avengers' minds. This would create an interesting plot where Steve has to find a way to make them remember him again.
The plot could develop in a way that Steve has to earn their trust again. He might start by helping the Avengers in small ways without revealing his true identity at first. As he does this, he could gradually form new relationships with the team members. However, some of the Avengers might be more difficult to convince than others, like Tony Stark who might be very skeptical.
Miscommunication. The Avengers could receive wrong information about Steve's actions or intentions, leading them to believe he's a danger and thus hurt him.
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.
Well, in some fanfictions, it could be due to Steve's old - fashioned values. Maybe he made some comments that were misinterpreted as being against homosexuality. For example, if he was overly strict about traditional relationships based on his own era's norms, the Avengers might wrongly assume he was homophobic.
Well, some of them might be really surprised. Tony, for example, would probably crack a lot of jokes about Steve being young. He might say things like 'Cap, you were this cute little thing?' Natasha, on the other hand, would be more observant and start thinking about how this new information could be used in future strategies. Thor would probably just laugh heartily and pat Steve on the back, saying something like 'Tis an interesting discovery, young Steve!'
Maybe it could be due to some of Steve's old - fashioned values. In the original setting, Steve is from an earlier era, and his traditional views might be misinterpreted as homophobic in the context of a fanfiction. For example, his conservative attitude towards relationships in general could be twisted to seem like he has negative views towards homosexuality.
Well, they might be really surprised. Some of them could be confused at first. Tony might make a snarky comment like 'What's going on here, boys?'. Natasha would probably just raise an eyebrow and wait for an explanation. And Thor might not fully understand the situation but would try to lighten the mood with a joke.
I really like 'Bucky's Redemption'. In this fanfic, Bucky is on a journey to fully come to terms with his past actions while Steve is there to support him every step of the way. It shows their loyalty to each other in a very touching way. There are some intense emotional moments as Bucky struggles with his guilt and Steve tries to pull him out of the darkness.
One great Steve Rogers Avengers fanfiction is 'The Captain's Heart'. It beautifully explores Steve's character post - Avengers events. It delves into his relationships with his teammates in a very realistic way. Another good one is 'Steve Rogers: A New Era'. This fanfic takes Steve into new and exciting adventures, while still staying true to his core values. And 'Avengers: Steve's Redemption' is also worth reading. It shows Steve dealing with the consequences of his actions in a really engaging manner.
One great 'Avengers Steve and Natasha fanfiction' is 'The Red and the Blue'. It beautifully explores their relationship in the context of their Avengers duties. The author does a wonderful job of showing their mutual respect and the unspoken connection between them.