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.
I'm not sure. Fanfiction can be a very niche and subjective area. Some people might find such a plot interesting in a 'what - if' scenario exploration within the Percy Jackson fandom, but it also goes against the typical relationship dynamic between Percy and Annabeth. It really depends on individual writers and what they're trying to convey with such a shocking plot device.
One possible motive could be to create a really dark and edgy take on the characters. Maybe the writer wants to explore how far the characters could be pushed out of their normal personalities. For example, if Percy was under some kind of dark magic influence, it could lead to such an out - of - character action.
The slap could also be a turning point in their relationship. It might make Percy more aware of Annabeth's feelings and boundaries. He could start to be more careful with his actions and words around her. On Annabeth's side, she might feel a bit guilty after the slap and try to make amends in her own way, like by doing something nice for Percy that she knows he'll appreciate.
However, it could also create a rift. Percy might feel unjustly treated if he didn't think he deserved the slap. This could lead to him pulling away from Annabeth for a while, causing tension and distance in their relationship. They would then have to overcome this obstacle to get back to where they were before.
In a large number of Percy and Annabeth fanfictions, they are indeed presented as soulmates. Their shared adventures, the way they understand each other without many words, and the deep loyalty they have for one another all contribute to this perception. Fanfiction authors often build on their existing relationship from the original works and enhance it to the level of soulmates. They might show it through things like Percy being able to sense Annabeth's distress from afar, or Annabeth always knowing exactly what Percy needs in difficult situations. This soulmate - like relationship makes for very engaging and heart - warming stories in the fanfiction realm.
It could be that Annabeth has a misunderstanding about Percy. She might think he's a threat to her position or status at the camp, so she bullies him to try and keep him in check.
It could be Chiron, the centaur. He's always keeping an eye on the campers and would likely be concerned if he caught them up to something. He might give them a stern talking - to and some extra chores as punishment.
Percy might be a bit flustered and embarrassed. He's protective of his relationship with Annabeth, so having Magnus walk in might make him feel like their privacy has been invaded. He could quickly try to shield Annabeth or just look really shocked.
Their shared experiences play a big role. They've been through so much together, like countless battles and adventures. Percy has seen Annabeth in tough situations and he knows how strong she is, but also how vulnerable. So he feels the need to protect her. For instance, they might have had a close call in a previous adventure where Annabeth was nearly killed, and that memory makes Percy extra vigilant.