Perhaps it's part of a grand plan that Poseidon has, one that he believes will ultimately benefit Percy but in the short - term causes him pain. For example, he might be trying to toughen Percy up or make him prove himself in a more extreme way, thinking it will help Percy in the long run.
In certain fanfictions, Poseidon might hurt Percy's reputation. He could do something that makes the other demigods look at Percy in a different, negative light. This could be by making decisions that seem to favor Percy too much, causing jealousy among others, or by getting Percy involved in some political mess within the godly realm that Percy has no control over. This can lead to Percy being ostracized or having to deal with a lot of unnecessary drama and pain.
One possible reason could be power politics among the gods. Poseidon might see Percy as a liability in some complex power play. Maybe other gods are pressuring him to distance himself from Percy for some hidden agenda. Another reason could be a misunderstanding. Percy might have unknowingly done something that Poseidon misinterprets as a threat to his domain or power. For example, Percy's actions in the mortal world might have unintended consequences for the underwater realm.
Well, one reason could be that fanfiction often takes creative liberties that might distort the original relationship between Poseidon and Percy in the official story. It could be that Poseidon is a very complex character in the original, and fanfiction simplifies or misrepresents him in a way that makes it seem like he 'hates' Percy in those fictional interpretations.
One possible theme could be family reconciliation. Since Percy and Poseidon have had a somewhat rocky relationship at times, a cuddle scene could symbolize them mending their differences and growing closer as father and son.
One reason is that some fanfiction authors may lack a deep understanding of Naruto's character. They might write things that are completely contrary to his values and personality just for the sake of creating a 'unique' story. For example, Naruto is known for his positive attitude and strong will, but in some fanfictions, he could be depicted as a negative or spineless character.
One reason is that batjokes may not fit the mood of the fanfiction. Fanfiction often has its own atmosphere and theme. If batjokes are too comical or out of place in a more serious - themed fanfiction, it can ruin the overall feeling. For example, in a dark and gritty Batman - related fanfiction, a silly batjoke can break the immersion.
One reason could be a false accusation. If Spiderman is wrongly accused of a crime by the Avengers or someone within the Avengers' circle, he might feel betrayed and lash out at them. He's just a young hero, and such a false accusation could really make him angry.
Misunderstandings play a big role. Maybe CC does something that lelouch misinterprets as betrayal, like making a deal with an enemy that lelouch doesn't understand at first. Also, in some fanfictions, lelouch's own insecurities about his role and the people around him could make him push CC away, which could be seen as a form of hurting her.
Miscommunication. The Avengers could receive wrong information about Steve's actions or intentions, leading them to believe he's a danger and thus hurt him.
One possible reason could be miscommunication. If Ahsoka misunderstands Anakin's intentions or vice versa, their actions might end up hurting each other. For instance, Anakin might give an order that Ahsoka thinks is wrong, and in her attempt to defy it, she accidentally causes him harm.