There are several possible reasons. Some fans might simply enjoy writing or reading about drama and relationship turmoil. Since Naruto has a large and diverse fan base, there will always be those who want to deviate from the traditional storylines. In this case, the idea of Sasuke cheating on Sakura with Karin could be a way for these fans to explore different character traits, relationships, and the consequences that would follow such a betrayal. It also gives them a chance to rewrite or re - imagine the characters in a more complex and perhaps darker light.
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.
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.
Some people might be exploring the darker side of human nature through these characters. They want to see how extreme situations can play out between Sasuke and Sakura.
I'm not entirely sure if it's a 'popular' trope. Some fans might be interested in exploring such dramatic and controversial storylines in fanfiction for the shock value or to explore different character dynamics. However, many fans of the original Naruto series who ship Sasuke and Sakura would likely be disappointed or even offended by such a concept.
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.
One reason could be that some fans want to explore the darker aspects of a character's life in a fictional and extreme way. They might think it adds depth or a different perspective to the character. But this is a rather controversial approach.
Well, some writers may be trying to create a more 'gritty' or 'realistic' version of the Naruto universe in their eyes. They might think that adding an element of abuse to Sakura's story can add more depth and drama. However, they may not fully consider the negative impact it can have on the character and the fans who love her. Also, it could be a way for the writer to express their own darker emotions or experiences through the fanfiction, although this is not a very healthy or appropriate way in this context.
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.
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.
In the realm of fanfiction, almost any scenario is possible. Regarding Sasuke divorcing Sakura for Karin, it may exist. Fanfiction authors draw inspiration from the source material and then take it in new directions. The idea might stem from Karin's past feelings for Sasuke and some fans' desire to see how such a relationship shift would impact the characters. It could also be a way to explore the characters' personalities more deeply, for example, how Sakura would cope with such a betrayal and how Karin would handle being with Sasuke in that context.