One reason could be the contrast between their characters. Percy is often seen as a heroic, adventurous type, while Artemis is a goddess of the hunt, more reserved. The idea of them as secret lovers creates an exciting 'opposites attract' scenario.
In some fanfictions, Percy might be depicted as Artemis' secret lover. Fanfiction allows for a lot of creative interpretations of characters and their relationships. However, in the original mythology and main series, this is not a canonical relationship.
Well, perhaps in this fanfiction, Artemis betrays Percy for Orion due to a prophecy. There could be a prophecy that states that Orion and Artemis together will bring about some great change or prevent a huge disaster, and Artemis believes she has to choose Orion over Percy to fulfill it. Also, Orion might have some sort of leverage over Artemis that forces her to turn her back on Percy.
One possible reason could be a misunderstanding. Maybe there was some miscommunication or false information spread that made Artemis think Percy had done something unforgivable. For example, if Percy was wrongly accused of harming one of Artemis' beloved creatures or desecrating her sacred places.
One possible reason could be a misunderstanding. Maybe Percy accidentally disrupted one of Artemis's hunts or sacred rituals in the fanfic. Another reason might be that some prophecy or fate in the fanfic's made - up world has pitted them against each other. For example, if there's a prophecy that Percy's actions will somehow limit Artemis's power or freedom in the future.
She might start to compare the real Percy (if he exists in the same universe) to the fan - created versions. This could lead to misunderstandings or unrealistic expectations.
One reason could be as a form of punishment. Maybe Percy did something that Artemis considered disrespectful to the female gender or to her values as a goddess. Another reason might be for a mission. Artemis could think that in a female form, Percy would be able to infiltrate a place or situation that is only accessible to females.
One possible reason could be differences in their divine natures. Percy is a son of Poseidon, a god associated with the sea and a more fluid, emotional element. Artemis is a virgin goddess of the hunt, very strict and unyielding in her ways. This contrast in their essences might lead to clashes and Percy's dislike.
It could be due to a lack of information. Artemis might not have known all the facts about Percy. For instance, if there was some sort of illusion or false information spread around, she could have misjudged the situation and accidentally killed him. Also, perhaps in the heat of a battle, she didn't have time to properly identify her target and made a tragic mistake.
One possible plot could be Percy getting caught in one of Artemis' hunts by accident. They initially clash due to their different natures but gradually develop feelings for each other.
It could be because Percy is different. If he comes from a different background or has a unique personality trait, the bullies might target him. In the fanfiction, if Percy is from a different race or has an unusual way of speaking, the bullies might pick on him for not being like them.