In some fanfictions, it could be that Percy has new, uncontrollable powers that the camp doesn't understand. They see him as a threat and banish him. For example, if he starts to show signs of a power that was once associated with an enemy of the camp. It's also possible that Percy's loyalty is questioned. If he befriends someone from an opposing group, the camp might think he has turned against them and banish him.
A common scenario in fanfiction is that there is a power struggle within the camp. A new leader or group comes to power who doesn't like Percy or feels threatened by his popularity and strength. So, they make up an excuse to banish him. This could also be related to a new set of rules or ideologies in the camp that Percy doesn't conform to, leading to his banishment.
In some fanfictions, Percy is heartbroken. He has considered the camp his home for so long, and being banished makes him feel lost and betrayed. He might go into a period of self - doubt, wondering if he really is as bad as they say.
In some fanfictions, Percy might be banished because he goes against the traditional ways of the demigod society too much. For example, if he tries to form an alliance with a group that the other demigods consider enemies. His actions could be seen as a threat to the stability of their world, so they banish him to keep the peace.
Well, one reason could be that he's wrongly accused of a major crime against the gods. Maybe some false evidence is planted, making the gods believe he's a threat, so they banish him. Another reason might be that he's trying to protect someone or something, and his actions are misinterpreted as going against the gods' will. For example, if he's trying to save a powerful but 'forbidden' creature, the gods might see this as a violation and send him to Tartarus.
One popular 'Percy Jackson Banished' fanfiction might be 'The Banished Hero's Return'. It often explores Percy's journey after being banished, facing new challenges and perhaps reuniting with old friends in unexpected ways. Another could be 'Banished to the Shadows' which delves into the darker aspects of Percy's banishment and how he copes with being an outcast in the world he once knew.
One possible reason could be that he feels misunderstood. In the camp, everyone has high expectations of him as a son of Poseidon. If he makes a small mistake, it might be blown out of proportion. For example, if he accidentally causes a small flood during a training session, instead of being understood as still learning control over his powers, he could be harshly criticized. Another reason might be that he's tired of the constant danger at the camp. Every day there seems to be a new threat looming, whether it's a monster attack or some sort of internal camp conflict. He might just want to get away and have some peace for a while.
He might leave because of a personal conflict. Perhaps he has a falling - out with some of the other campers or with the camp leaders. Like if he disagrees with a new rule or policy that the camp has implemented which he thinks is unjust.
In some fanfictions, he might be exiled because of false accusations. Maybe someone frames him for a crime he didn't commit, like stealing a powerful artifact from the camp's vault. The camp leaders, believing the false evidence, have no choice but to exile him.
It could be Ares. He is often portrayed as a violent and unpredictable god. Percy might banish him if he goes too far in causing wars and chaos among mortals. His banishment would have a huge impact on the battlefields of the mortal world.
One possible fanfiction could be where Percy and Annabeth are banished due to some false accusations in the demigod community. Maybe they are wrongly blamed for a powerful magical artifact going missing, and as a result, they are forced to leave Camp Half - Blood and go on a journey to prove their innocence. They might face many challenges on their own, away from their friends and the protection of the camp.
One reason could be if he was wrongly accused of stealing something important from the camp. Maybe a powerful magical item goes missing and Percy is the prime suspect because of his past issues with authority. Another reason might be if he accidentally endangers the camp in some way during a mission or training exercise. For example, if he unknowingly leads some dangerous monsters back to the camp.