One reason could be that she made a hasty decision during a battle. For example, she might have led her team into a dangerous situation without proper planning, and as a result, some of her comrades got injured. So she would need to apologize for her poor leadership in that instance.
One reason could be that she realizes she has been too dismissive of Naruto's feelings for her. She might have led him on or been cruel in rejecting his advances in the past. Another reason is that she finally understands how much Naruto has sacrificed for the village and for her personally, and she feels guilty for not showing him more respect and gratitude earlier.
Maybe she broke her mother's favorite vase while playing. It was an accident but she knew it was important to her mother.
She might have made false accusations against the people around her love interest due to her insecurities. When she found out the truth, she felt ashamed of herself. So she apologized to make up for the trouble she had caused. Also, if she had tried to isolate her love interest from his friends and family, she could apologize when she understood how wrong it was.
In a story, a girl might need to apologize to her mother if she failed to keep a commitment. For instance, if she promised to help with the household chores but didn't do it. Or she could have disobeyed her mother's instructions, such as not coming home on time. Also, if she gossiped about her mother to others, that would be a reason to say sorry.
Perhaps Annabeth made a decision without consulting Percy that put him in danger. In the fanfiction world, this could lead to her feeling guilty and apologizing. For example, if she led their group into a trap because of her over - confidence, she'd realize her mistake and say sorry to Percy.
In a story, a girl might apologize to her mother if she failed to keep a promise. Say, she promised to study hard for an exam but instead she went out with friends and got a bad grade. Or, she could have borrowed money from her mother and spent it on something frivolous instead of what she said she would use it for. Additionally, if she had been rude to her mother's friends or guests, she would surely need to apologize.
Hiccup's over - confidence could also make Astrid mad. If he goes into a dangerous situation without proper preparation, thinking he can handle it because of his previous experiences, and then things go wrong. Astrid, who believes in careful planning, would not be happy with his reckless behavior.
One reason could be that the fanfic changes the essence of their relationship. In the original, they have a certain dynamic that she values, and fanfics might mess it up.
One possible reason could be that she might have underestimated Naruto in the past. She often belittled his abilities and dreams, but as she grew and realized his true strength and determination, she felt the need to apologize for her previous behavior.
There could be a mean - spirited colleague named Mark. He might constantly make snide remarks about Astrid's work or appearance, which would add to her discomfort. Another character could be a nosy neighbor, Mrs. Thompson. She always pries into Astrid's personal life, asking inappropriate questions and making Astrid feel awkward. And then there could be a clumsy acquaintance, Jake, who always seems to get in Astrid's way and cause embarrassing situations for her.