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.
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.
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.
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.
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.
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.
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 he feels a sense of being out of place. Maybe new campers have come in and he no longer feels as important or needed. He might also leave in search of a greater purpose. In some fanfictions, he could be on a quest to find something that he believes is crucial for the safety of the camp, but he has to leave the camp itself to do it.
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.