Maybe he feels betrayed by the Olympians. They often have their own agendas and don't always consider the well - being of demigods like Percy. For example, they might send him on dangerous quests without proper support.
Another reason could be related to his personal relationships. If his friends or family have been wronged by Olympus in some way, Percy's loyalty towards them might override his loyalty to Olympus. He is a very emotional person, and if someone he cares about is hurt by the Olympian powers, he might turn against them.
His friends might be divided. Some who are more inclined to support Olympus might be at odds with Percy. For example, Annabeth, who has a certain respect for the gods, could face a dilemma in choosing between Percy and Olympus.
In fanfiction, the politics and intrigues within Olympus might be depicted in a way that makes Percy disgusted. The backstabbing and power - hungry nature of some of the Olympians could be too much for him to bear, so he hates Olympus.
One possible reason could be that Percy feels betrayed by the Olympians first. Maybe he discovers some dark secrets about them that make him lose faith. For example, if he finds out that they have been using demigods unfairly in their battles or politics.
One possible reason could be to create a tragic story. Writers might want to show how the world of the story would be affected by the loss of such a central character like Percy.
Maybe he accidentally breaks some ancient and unforgivable rule. For example, he could have uncovered a secret that the gods wanted to keep hidden and in their attempt to silence him, they banish him. In fanfiction, the writers often play with the idea of Percy's curiosity getting him into big trouble with the Olympians.
Maybe he's tired of the gods' constant interference and manipulation in his life. They often use him for their own purposes without much regard for his well - being.
One possible reason could be past trauma. Maybe he had a bad encounter with a man in the past, like a bully or an authority figure who mistreated him. This could have left a deep scar, making him terrified.
One reason could be that he was framed for a crime he didn't commit and had to run away to clear his name. In the complex world of the fanfiction, there could be a conspiracy against him. Some powerful beings might not want him to be with Annabeth or have a child, so they set him up and made him disappear.
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.