In some fanfictions, external forces could be the cause. Maybe Percy is under some sort of spell or mind control that makes him act out against Annabeth. Or perhaps there's a plot where he believes that by bullying her, he can protect her from some greater danger, although his method is clearly wrong.
One possible motive could be miscommunication. In fanfiction, if there's a misunderstanding between them, Percy might lash out and seem like he's bullying Annabeth. For example, if he misinterprets her actions as a betrayal, he could start being mean to her.
One possible motive could be protection. Annabeth might think that there is a threat to Percy that she can only handle by taking him away from the situation. For example, if there's a prophecy that puts Percy in danger and she believes she can keep him safe in a hidden location.
One possible motive could be to create a really dark and edgy take on the characters. Maybe the writer wants to explore how far the characters could be pushed out of their normal personalities. For example, if Percy was under some kind of dark magic influence, it could lead to such an out - of - character action.
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.
Perhaps in the fanfiction, Bella has been influenced by a darker force. It could be a new vampire or some other supernatural entity that has turned her into a bully. This external influence could be warping her mind and making her act out against those she once cared about or new people in Forks.
There are fanfictions that explore the idea of Percy bullying Annabeth. This can be a shock to fans who know the original characters well. The reasons for such a portrayal in fanfiction can vary. It could be to create drama, to explore a 'what - if' scenario where their relationship takes a dark turn. Maybe the writer wants to show how Annabeth copes with such difficult situations, but overall, it's not a common or expected portrayal given their canonical relationship.
Another motive could be a difference in ideology. If Percy believes in a different way of handling things compared to Artemis, and he feels that her methods are endangering the world or his loved ones, he might decide to betray her in order to stop her from following her current path.
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.
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.
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 possibility is that Annabeth challenges Percy to a little competition related to the field trip. For example, if they are at a museum, they could see who can find the most interesting historical fact about an exhibit first. This would be a fun and friendly way for them to interact and also show off their knowledge.