One reason could be a misunderstanding. Maybe Kate did something that Tony misinterpreted, leading to him telling her off. For example, if she made a decision without consulting him in a shared project within the story's universe.
It could be due to jealousy. If there's a third character involved, say, Kate gets closer to someone Tony has feelings for, he might tell her off out of envy. In fanfiction, relationships and emotions are often explored in complex ways. It might also be that Kate has violated some unspoken rule or code in their group or community in the story, so Tony has to confront her.
Perhaps Kate did something that went against the team's protocol. For example, she went solo on a lead without informing the team. Tony, who values teamwork highly, tells her off. He would likely remind her of how important it is to work together and follow the rules in NCIS.
Well, another option is to look for fanfiction that involves characters named Tony and Kate in a confrontational situation. You can use more general search terms like 'Tony and Kate conflict fanfiction' on search engines dedicated to fan works. This might lead you to something similar to what you're looking for.
It could strain their relationship at first. Kate might feel offended and become a bit distant from Tony. She might think he was too harsh in his telling - off.
One reason could be Hermione's excessive worrying. She often frets over small details and Harry might get tired of it and tell her off. For example, if she keeps nagging about the proper way to perform a spell when they are in a hurry during a dangerous situation.
It could also be due to external pressure. The media or some powerful figures in the fanfiction world might force Tony to stay away from Peter. They could spread false rumors about their relationship or use their influence to separate them for their own gain.
Sakura's obsession with Sasuke. Naruto might be fed up with how Sakura always puts Sasuke first, ignoring the people around her who truly care about her, like Naruto himself. So he tells her off to make her realize her wrong priorities.
One possible reason could be a misunderstanding. Maybe Peter overheard something Tony said and misinterpreted it as a sign of mistrust or dislike. For example, if Tony was talking to another character about Peter's abilities in a way that Peter thought was critical rather than constructive.
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.
A good Tony and Kate married fanfiction could also delve into their emotional connection. Like how they support each other through tough times. Maybe one of them has a family issue or a problem at work that the other helps them get through. It could show their growth as individuals and as a couple. It could also include some sweet moments like anniversaries or surprise dates that they plan for each other.
There could be several reasons. Maybe the writer is trying to create conflict. Cheating is a very effective way to introduce a lot of drama into a story. It can also be a way to explore different character personalities. For example, showing Garth's weakness or Kate's strength in dealing with such a situation. And perhaps the writer just has a unique take on these characters and wants to see how they would interact in this rather scandalous situation.