One possible reason could be her deep love for Percy. She might see his life as more valuable in the grand scheme of things, perhaps because of a prophecy or a mission that only he can complete. So, she chooses to sacrifice herself to ensure his survival.
It might also be related to some sort of enemy or threat that specifically targets Percy and his relationship with Annabeth. The enemy could be so powerful that Percy realizes the only way to stop it from harming Annabeth is to sacrifice himself, like if the enemy feeds on his life force or his connection to Annabeth, and he chooses to end it by sacrificing his own life.
One possible reason could be that she sacrifices herself to save someone else. Maybe she jumps in front of a fatal blow meant for Percy or one of the other important demigods. Another reason could be that she makes a miscalculation in her battle strategy. She might underestimate the strength or the trickery of the giants and end up in a situation where she can't escape.
Love is a big reason. She loves Percy deeply and is afraid of losing him. In the world of the fanfiction, there could be many threats to their relationship, so she holds on tightly.
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.
It could also be due to some sort of magic or mind - control. A powerful enemy might have cast a spell on Annabeth, forcing her to turn against Percy. In this case, it's not really her own will but the result of external manipulation.
Maybe she's scared because the fanfiction portrays Percy in a way that's out of character for him. Like, it might show him being overly aggressive or having strange powers that she doesn't recognize, which makes her feel like she doesn't really know him anymore.
It could also be that in the fanfiction, Percy is given a new power or ability that Annabeth sees as a threat. Or perhaps Percy is made to side with someone Annabeth dislikes, causing her to 'hate' him in that fan - created story. This shows how different plot elements in fanfiction can change the dynamics between characters.
Perhaps the fanfiction is set in an alternate universe where something has happened to change Percy's feelings towards Annabeth. For example, if there was a betrayal in the fan - created story that made Percy turn against her.
One possible reason could be a misunderstanding. Maybe she saw Percy doing something that she misinterpreted as a betrayal of their friendship or their cause. For example, if Percy was seen talking secretly with an enemy, Annabeth might jump to conclusions without knowing the full story.
One possible scenario could be that Percy sacrifices himself to save Annabeth from a powerful and deadly monster. Maybe a huge and ancient sea creature that only Percy can hold off long enough for Annabeth to escape, but at the cost of his own life.