One possible reason could be Sasuke's continuous absence. He is often away on missions, leaving Sakura feeling lonely and neglected. In fanfiction, this could be exaggerated to a point where she can't take it anymore.
The other members of their team or the village might have mixed reactions. Some might side with Sakura, understanding her point of view, while others might think she's being hasty or that they should try to preserve the relationship for the sake of the village's stability or their own friendship circle.
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 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.
For their friends, it can be a huge shock. Naruto, for example, who has always been in the middle, trying to keep their relationship strong, might feel a sense of failure. Their other friends from the ninja teams might be divided in loyalty, some siding with Sasuke, some with Sakura. In the village, it could cause a scandal. The villagers might gossip and it could also affect the image of the Uchiha clan again, as Sasuke is part of it.
One possible reason could be Sasuke's continuous absence due to his solo missions. He's always away, leaving Sakura feeling lonely and neglected in their relationship. This lack of emotional and physical presence can gradually erode the bond between them.
It would definitely shock their friends. Naruto, for instance, would be caught in a difficult position as he is a close friend to both. He would probably try to mediate but might not be successful.
A fanfic could depict their separation and divorce in a very emotional way. For instance, Sakura might have found out some secrets about Sasuke that she couldn't accept. This led to a huge fight between them, and they separated. The divorce would then be a formal end to a relationship that had once held so much promise. It could show how their friends reacted to this news, some being supportive of Sakura's decision while others trying to get them back together.
In this type of fanfic, it could be that external forces play a role. For example, there could be a new threat to the village that Sasuke feels he must face alone. He distances himself from Sakura to protect her, but she misinterprets it as him not loving her anymore. This misunderstanding leads to their separation. As for the divorce, it could be a way for them to finally move on from each other, even though deep down they still have some feelings left.
Sakura may also try to find new friends and a support system. She could join a group of female ninja who have similar experiences or interests. They could encourage her to move on, and she might start to explore new hobbies or relationships, gradually getting over the divorce from Sasuke.