One reason could be stress. Maybe they are in a really tough situation in the fanfiction, like facing a powerful enemy. Annabeth, being under a lot of pressure, snaps at Percy if she thinks he's not taking things seriously enough. Another possibility is jealousy. If Percy gets a lot of attention from someone else, Annabeth might snap out of insecurity.
It could be that Annabeth has her own personal issues or fears that she's dealing with. She might be worried about her future, her family, or her own abilities. And Percy, perhaps unknowingly, does or says something that triggers her. For instance, if he accidentally reminds her of a past failure or a fear, she could lash out at him.
In some fanfictions, miscommunication plays a big role. Percy might say something that Annabeth misinterprets. For example, he could make a joke about their relationship or their quest that she takes the wrong way. Also, if Percy makes a decision without consulting Annabeth first, especially if it's a risky one, she could snap at him. She values being part of the decision - making process.
It could also be due to some sort of magic or mind - control. A powerful enemy might have cast a spell on Annabeth, forcing her to turn against Percy. In this case, it's not really her own will but the result of external manipulation.
Well, one possible reason could be that Percy made a really stupid decision without consulting Annabeth first. Like, if he went off on a dangerous mission on his own when they were supposed to be a team. Annabeth is a very strategic person and she might feel betrayed or extremely frustrated by such actions.
One possible reason could be Percy's recklessness. He often jumps into dangerous situations without thinking fully, and Annabeth, being the more strategic one, might get frustrated and yell at him. For example, if Percy decides to face a powerful monster alone just to prove himself, Annabeth would be worried and angry at his lack of caution.
One reason could be miscommunication. Maybe Percy misinterpreted something Annabeth said or did. For example, if Annabeth made a decision about a quest without consulting Percy, he might get angry and yell.
Maybe Percy has shown some uncontrollable power that scares Annabeth.
One reason could be a shift in the plot where Annabeth's character development leads her to realize she and Percy are not compatible in the way they were initially thought. Maybe she has new goals or a different understanding of herself that makes her feel she's better off without Percy in the fanfiction world.
In some fanfictions, Annabeth could abandon Percy because of a prophecy. If the prophecy seems to imply that being with Percy will lead to a great disaster, she might choose to leave to protect him or the world. Also, it could be due to a personal growth arc for Annabeth. She may realize that she has been too dependent on Percy in the past and decides to go off on her own to find her true self, leaving Percy behind.
One possible reason could be a misunderstanding. Maybe Percy did something that Annabeth misinterpreted, like being overly friendly with another character which made her jealous and led to her rejection.
One possible reason could be a misunderstanding. Maybe she saw Percy doing something that she misinterpreted as a betrayal of their friendship or their cause. For example, if Percy was seen talking secretly with an enemy, Annabeth might jump to conclusions without knowing the full story.
One reason could be that Percy often gets into dangerous situations alone to protect others. Annabeth, being his friend or more, would feel bad seeing him constantly at risk.