Perhaps in this fanfiction's plot, Sakura has some sort of special inheritance or power-up. It could be that she has a connection to an ancient and powerful force. This power could be something that was dormant within her all along and finally surfaces, making her stronger than Sasuke. For instance, she might have a link to a long - forgotten sage or a special kekkei genkai that was previously unknown, which now gives her extraordinary abilities.
It could also be that the writer wants to subvert the traditional power dynamics. In the original series, Sasuke is often seen as very powerful. But in this fanfiction, Sakura might be stronger because she has a stronger willpower. She could be more determined to protect her loved ones and this gives her an extra boost in strength. For example, her love for the village and her friends drives her to train harder and unlock new levels of power.
Protectiveness. Sasuke has seen a lot of danger in his life. He knows Sakura is a valuable person, both as a friend and as someone he cares about. So his possessiveness could stem from his desire to keep her out of harm's way. He doesn't want any bad guys to get near her.
One possible reason could be that Naruto has a stronger willpower in the fanfic. His never - give - up attitude might be enhanced, allowing him to push past his limits more easily than Sasuke. For example, he might have an even deeper connection to the Nine - Tails' power, enabling him to draw on it more effectively in battles.
The relationship might become more equal. Since Sakura is stronger, Sasuke can't look down on her as he might have in the original story. They could become more like true partners, respecting each other's abilities.
One possible reason could be Sasuke's continued obsession with his revenge - even after they got married, his focus on past grudges might have driven a wedge between them. Another could be differences in their life goals. Sakura might have wanted a more settled family life, while Sasuke was still drawn to his wandering ways in the name of protecting the village from afar.
It could also be a way for the writer to express their own dissatisfaction with the original relationship between Sakura and Sasuke. Maybe they felt that Sakura's character was not well - developed in the context of that relationship and decided to create a more 'dramatic' storyline through cheating.
In fanfiction, Sasuke might snap at Sakura due to miscommunication. Sakura could be expressing her concern for him in a way that he misinterprets. For instance, if she tells him not to go on a dangerous mission and he thinks she is trying to control him instead of just caring about his safety. Another factor could be the influence of external forces. Maybe there are other characters or events that put him in a bad mood, and Sakura just happens to be there at the wrong time, so he snaps at her.
Sakura might cry over Sasuke because of the distance between them. In many fanfictions, Sasuke goes off on his own journey, leaving Sakura behind. She misses him and the pain of separation makes her cry. Additionally, if Sasuke gets injured severely in a fight, Sakura, being a medic, might cry out of worry for his well - being and the possibility of losing him.
One possible reason could be that Sasuke is too focused on his revenge. His single - minded pursuit of power and vengeance against Itachi leaves little room for romantic feelings. He might see a relationship with Sakura as a distraction from his goals.
Some writers might want to explore different character traits. They may think that Sasuke has a more complex personality that could lead him to make such a mistake. Also, it could be a way to show Sakura's reaction in a difficult situation and how she might grow or change because of it. In addition, it might be an attempt to break the traditional 'happy ending' mold of their relationship in the original story.
One reason could be to create drama. The idea of a relationship being broken by cheating is full of emotional turmoil which can make for an interesting read. Another reason might be some writers' preference for the Sasuke - Karin dynamic. They might see potential in their relationship that wasn't explored in the original series.