It could be due to jealousy. In fanfiction, there might be a new character who is getting too close to Annabeth, and Percy misinterprets their relationship, which makes him mad at her.
Well, in some fanfictions, Percy might be mad at Annabeth because of a misunderstanding. Maybe she made a decision without consulting him in a crucial situation regarding their adventures. For example, if she chose a path that put their friends in danger without his input, Percy could get angry.
Another reason could be that Annabeth broke a promise she made to Percy. Promises are a big deal in their relationship, and if she fails to keep one, like not being there when he really needed her, Percy would be understandably mad.
One possible reason could be a misunderstanding. Maybe Annabeth made a decision without consulting Percy, like choosing a quest path that Percy thought was too dangerous. Percy, being protective and having his own ideas about how things should be done, might get mad.
It could be because of a misunderstanding. In the fanfic, perhaps Annabeth was seen talking to an old acquaintance who Percy doesn't trust. He might jump to conclusions and get angry with her without really knowing the whole story. Like, if he thought she was plotting something behind his back with that person.
Well, in many fanfictions, Percy might be seen as too ambitious and career - focused. He could have made choices that put his job at the Ministry above his family. For example, he might have supported Ministry policies that the rest of the Weasleys disagreed with, like their stance on Harry Potter or how they were handling the growing threat of Voldemort. This could lead to the Weasleys being mad at him for seemingly choosing the wrong side or not standing by the family values.
When Annabeth is mad, Percy will likely try to make it up to her. He might start doing all kinds of things like bringing her favorite foods, trying to help her with her projects. But Annabeth might not be so easily swayed. She'll be looking for a real apology and for Percy to show that he's learned his lesson. So the relationship becomes a bit of a power struggle, with Percy trying to regain her favor and Annabeth deciding whether he's worthy of it.
One scenario could be that Annabeth accidentally revealed Percy's secret identity to someone he didn't want to know. This could put him in danger and make him furious with her.
Perhaps Percy forgets an important promise he made to Annabeth. In their relationship, promises are significant. If he fails to keep one, like a promise to help her with a project or be at a certain place at a certain time, Annabeth would surely be mad. It could also be that Percy accidentally breaks one of Annabeth's creations or possessions, which she holds dear.
Perhaps Percy made a decision without consulting Annabeth. In many fanfictions, Annabeth values being part of the decision - making process, especially when it concerns their relationship or a mission they're on. If Percy went off and did something like made a deal with a potentially dangerous character on his own, Annabeth would be furious.
Let's say Annabeth put Percy's friends in danger without his knowledge. Percy is very loyal to his friends, and knowing that Annabeth's actions could have hurt them would surely make him angry at her.
It could be that Annabeth has a misunderstanding about Percy. She might think he's a threat to her position or status at the camp, so she bullies him to try and keep him in check.
It can strain their relationship. When Annabeth is mad, there will be tension between them. They might not communicate as effectively, which could lead to misunderstandings. For example, if Percy doesn't understand why Annabeth is mad, he might say or do the wrong things, making the situation worse.