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.
Well, one reason could be that in the fanfiction, the writer might want to explore a different dynamic. Maybe Annabeth gets lured by Luke's charm that has been newly developed in the fanfic. Luke could be presented as more understanding or having a different kind of allure that attracts Annabeth away from Percy.
One reason could be that she's scared. The idea of becoming a mother at that point in her life might be overwhelming, and she might think she can't handle it with Percy. Maybe she wants to protect Percy from the difficulties that come with the pregnancy as well.
Perhaps there are external factors at play. The world of demigods is full of chaos and danger. Reyna might be in a position where Percy feels he has to be by her side to protect her or to fulfill some greater prophecy that involves the two of them, and this leads to him leaving Annabeth.
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.
Perhaps Annabeth made a decision without consulting Percy that put him in danger. In the fanfiction world, this could lead to her feeling guilty and apologizing. For example, if she led their group into a trap because of her over - confidence, she'd realize her mistake and say sorry to Percy.
Maybe he was hit by some magic or a spell that affected his memory. It could also be due to some sort of trauma he experienced that made his mind block out his memories of Annabeth.
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.
Maybe she's scared because the fanfiction portrays Percy in a way that's out of character for him. Like, it might show him being overly aggressive or having strange powers that she doesn't recognize, which makes her feel like she doesn't really know him anymore.
Well, perhaps in the fanfiction, Annabeth could initially be in denial about being pregnant after Percy leaves. But as time goes on, she starts to accept it and decides to focus on building a better future for her and the baby. She might also get support from her friends like Grover, who could help her out in various ways, such as finding a safe place to stay and providing emotional support.