She might be jealous. Maybe Naruto has achieved something that Sakura has been striving for, like getting recognition from a certain person or mastering a new jutsu. Her jealousy could get the better of her and lead to her lashing out and injuring Naruto.
One possible reason could be for the sake of creating a more complex love triangle in fanfiction. Maybe the writer wants to explore a different dynamic where Naruto and Hinata's relationship is strengthened by having a common dislike for Sakura. It could also be that the fanfiction writer is trying to give Hinata more agency and make her a stronger character by having her oppose Sakura in some way, perhaps in matters related to Naruto's affections.
In some fanfictions, Naruto might be influenced by an external force. Maybe a villain has the ability to manipulate emotions, and they target Naruto, making him lose control and hit Sakura back. This then sets up a story arc where Naruto has to overcome this external influence and make amends with Sakura.
It could be that in the fanfiction, Naruto's character is developed in a way that Sakura, within the sasunaru concept, doesn't approve. For instance, if the fanfiction makes Naruto too weak or too different from his original self. Also, if the fanfiction focuses too much on certain relationships that she doesn't like, she would hate it.
In some fanfiction, it could be related to a misinterpretation of character relationships. Maybe the author has a skewed view of Naruto and Sakura's dynamic and decides to create a tragic and unexpected turn of events where Naruto kills Sakura to show the breakdown of their bond.
Well, Naruto has a very warm and persistent personality. He's always there for his friends, and he has a big heart. In fanfiction, Sakura might see these qualities in Naruto and realize his true value. He's the kind of person who can make her laugh and feel safe, which could be very appealing compared to Sasuke's more brooding nature.
One possible reason could be that in some fanfiction, Naruto might be written to hate them because of their early treatment of him. Sakura was often dismissive of Naruto in the beginning, and Ino was more focused on Sasuke too. So in a different narrative, Naruto could hold grudges against them for not seeing his worth initially.
In the Naruto fandom, such a fanfic would be very out of character for both Naruto and Sasuke. They are comrades and friends with Sakura. However, some dark or AU (Alternate Universe) fanfics might explore extreme and unorthodox ideas. But in the main canon, they would never do this as they have a deep bond with Sakura. Naruto has been friends with her since childhood and Sasuke, despite his aloofness, also values her in his own way.
Sakura might be completely shocked at first. She might freeze for a moment, not believing what's happening. Then she could start crying tears of joy and say yes immediately.
Maybe she's a very private character. For instance, if Sakura values her privacy a great deal in the original story, fans might feel restricted in writing fanfiction about her personal life or thoughts. So, she 'holds back' the fanfiction in that sense.
Sakura's ability to forgive and still care for Sasuke despite his past actions could also make him proud. In many fanfictions, she represents unconditional love. This selfless love and her ability to see the good in him, even when he was at his worst, could make Sasuke proud of her as a person.