One possible reason could be that Karin has unique abilities that are more useful to Sasuke in his goals. For example, her sensing abilities might be crucial in his missions. Also, Karin has a deep loyalty towards Sasuke which he might value more than Sakura's feelings at that time.
There are fanfics like that. Sasuke might leave Sakura for Karin because of Karin's loyalty. Karin has shown unwavering loyalty to Sasuke throughout the series. She was willing to risk her life for him on multiple occasions. In contrast, Sasuke might think that Sakura's love is more of a childhood crush that she hasn't fully grown out of, and he may be drawn to Karin's more mature and selfless loyalty.
One reason could be that Sasuke realizes his true feelings for Karin were always there. Maybe in the fanfiction, he had some sort of epiphany that his connection with Sakura was more of a convenience or misunderstanding, while his bond with Karin was deeper on an emotional level.
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.
It changes the relationship between Sasuke and Sakura significantly. Instead of the canonical relationship where they end up together, Sakura is left heartbroken. This could also lead to a change in Sakura's character, perhaps making her more independent. As for Sasuke and Karin, their relationship becomes more central and they might face different challenges together as a couple.
One possible reason could be his long - standing feelings for Sakura. Despite his journey and time with Karin, his original connection with Sakura might have resurfaced. Sasuke has known Sakura since they were young, and those early memories and emotions could have influenced his decision.
One possible reason could be that Sakura has finally grown tired of Sasuke's aloof and cold behavior towards her. She might have realized that she has been constantly chasing after him while getting little in return, so she decides to ignore him as a form of self - preservation.
It could be due to some fans' different interpretations of the characters. Maybe they see a certain chemistry between Sasuke and Karin in the original series that they want to explore further, even if it means creating a scenario where Sasuke cheats on Sakura. Also, fanfiction is a place for creativity and exploring different possibilities, and this is just one of the many extreme or different ideas that can come out of that.
Perhaps Sasuke was testing Sakura's loyalty or strength. He may have thought that if she couldn't withstand his aggression, she wasn't worthy of being in his life. Also, his own inner demons, like the hatred he carried from his past, could have flared up and led him to almost kill her without fully realizing what he was doing.
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 was under some kind of mind - control or dark influence at that moment. Maybe he was influenced by an enemy's jutsu that made him lose his sense of self and turn on Sakura.