There could be some misunderstanding between them. Maybe Percy did something that Annabeth misinterpreted, and instead of talking it out like normal people, she decided to bully him. For example, if Percy accidentally took credit for something that Annabeth thought was hers, it could start this negative behavior.
Percy could stand up to Annabeth. He's a strong character. He might confront her directly about her bullying behavior and tell her it's not okay. This could lead to a turning point in their relationship, where Annabeth realizes she's been wrong.
One possible reason could be that the fanfiction misrepresents their characters. Maybe it portrays Percy in a way that Annabeth doesn't like or goes against the essence of their relationship as she sees it.
One possible reason could be the long - standing rivalry between Gryffindor and Slytherin. Ron, as a Gryffindor, might see Draco, a Slytherin, as a symbol of all the things he dislikes about Slytherin house, like their ambition being perceived as self - serving and their often haughty attitudes. Another reason could be personal grudges. Maybe Draco has said or done things to Ron's friends, like Hermione or Harry, and Ron is standing up for them in a rather bully - like way instead of handling it more maturely.
Maybe she's jealous of Percy's popularity at school. It could be that he always gets more attention, whether it's from teachers or other students.
One possible consequence could be Percy feeling betrayed and heartbroken. His trust in Annabeth would be shattered, and he might become withdrawn or angry.
It could also be a matter of self - discovery. Annabeth might have been on a journey to find out who she was outside of her relationship with Percy. Meeting this mortal was a turning point where she realized she wanted to explore a different path, even if it meant leaving Percy. Maybe she thought that being with a mortal would give her a chance to have a more ordinary life, at least for a while.
It could also be that in the fanfiction, Percy is given a new power or ability that Annabeth sees as a threat. Or perhaps Percy is made to side with someone Annabeth dislikes, causing her to 'hate' him in that fan - created story. This shows how different plot elements in fanfiction can change the dynamics between characters.
Perhaps the fanfiction is set in an alternate universe where something has happened to change Percy's feelings towards Annabeth. For example, if there was a betrayal in the fan - created story that made Percy turn against her.
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.