Perhaps Sasuke starts to notice Ino's unique skills and how they complement his own. Ino has her mind - related jutsus which could be useful in certain missions or battles. This practical aspect, combined with a growing personal bond, might lead him to leave Sakura for Ino.
One possible reason could be his strong bond with Naruto. Their relationship has been through so much, and Sasuke might realize that Naruto understands him on a deeper level than Sakura. Maybe he feels that his journey and growth are more intertwined with Naruto's, and he values that connection more in this fanfiction.
One possible reason could be Sasuke's pursuit of power. In many fanfictions, his thirst for greater strength might lead him to leave Sakura behind as he embarks on a solo journey to train in more dangerous and secluded places. He may feel that his goals are too dangerous for her to be involved with.
One possible reason could be his sense of duty. Sasuke has always been driven by his goals for the village or his personal vendettas. Maybe he feels that his mission is more important than staying with Sakura and their unborn child, believing that his actions are for a greater good that will ultimately protect them as well.
She may initially be angry. Sakura is a proud and strong character. She could confront Sasuke and Ino, demanding an explanation. After that, she might decide to become even stronger, both in her ninja skills and her emotional resilience, to show Sasuke what he's missing.
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.
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 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.
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 could be due to a misunderstanding. Sasuke might have thought Sakura was involved in something harmful to him or his goals. For example, if there were false rumors about Sakura betraying him, his hot - tempered nature in the Naruto series could lead him to this extreme reaction without fully understanding the truth.
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.