Well, maybe Percy realized they had different life goals. For example, if he was really into a life of adventure and she was more of a homebody. Another reason could be that he fell for someone else in the fanfic world, perhaps a new character who shared his interests more closely.
In the fanfic, it might be that their personalities just clashed over time. At first, they might have been attracted to each other, but as they got to know each other better, their differences became more pronounced. For instance, if Percy was a very logical person and Rachel was more emotional, it could lead to constant arguments and ultimately, Percy deciding to end the relationship.
It could be due to miscommunication. In fanfics, this is a common cause of relationship breakdowns. Maybe Rachel said something that Percy misinterpreted, and it led to a growing rift between them. Or perhaps there were outside forces at work, like Percy's family or friends didn't approve of Rachel for some reason, and he caved under the pressure.
On the other hand, she might try to be the bigger person. She could accept the break - up gracefully and focus on self - improvement. Maybe she starts taking up new hobbies or focusing more on her career or studies in the fanfic, using the break - up as a chance to grow and become a better version of herself.
Well, perhaps in the fanfic, there's a prophecy that gets misinterpreted. The gods think Percy is going to cause great harm based on this wrong interpretation, so they send him to Tartarus. It could also be that there's some sort of power struggle among the gods. Percy might be caught in the middle of it, and one side decides to get rid of him by banishing him to Tartarus to tip the balance in their favor.
One reason could be that Rachel represents a different world to Percy. She's more of an outsider to the demigod world, and Percy might be attracted to that sense of newness and freedom she seems to offer. Maybe he's tired of the constant danger and drama in the demigod life with Annabeth.
One possible reason could be jealousy. Maybe Rachel has something that Annabeth wants, like a closer relationship with a particular character. For example, if they both like the same guy, Annabeth might hate Rachel out of envy.
One possible reason could be a misunderstanding. Maybe Annabeth made a decision without consulting Percy, like choosing a quest that she knew was dangerous but didn't tell him until it was too late. Percy might feel left out and mad about it.
One possible plot twist could be that Percy is already in love with someone else, and Rachel's flirting makes things really complicated. He might be torn between his feelings for the other person and the new attention from Rachel.
One possible reason could be Percy's recklessness. He often jumps into dangerous situations without fully thinking them through, and Annabeth, being the more strategic one, might get frustrated and yell at him for endangering himself and others.
One reason could be that he values his mortal connections. His relationships with his friends and family are deeply rooted in the mortal world. Immortality might separate him from them, as they would age and die while he remains unchanged. For example, his bond with his mother is very strong, and he wouldn't want to lose that human connection.
One reason could be his heroic nature. Percy has accomplished many great deeds, and Aphrodite might be attracted to his courage and selflessness. Another could be his unique connection to the water, as Aphrodite has an appreciation for beauty and Percy's control over water could be seen as a beautiful and powerful thing in her eyes.
Well, it could be about different understandings of Percy. Annabeth knows Percy as a demigod, as a hero in their world. Rachel, on the other hand, might see Percy in a different light, more as an individual outside of the demigod stuff. They might fight because they both think they know what's best for Percy and how he should be. Also, it could be a fight for Percy's future. Annabeth might want Percy to stay in the demigod world and continue their adventures together, while Rachel could have different plans for him.