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.
Perhaps in this fanfiction, Sasuke is on a path of self - destruction. He might be trying to push away everyone who cares about him, including Sakura. By putting a knife against her, he's not only showing his aggression but also his inner turmoil and the fact that he believes he's too far gone to be redeemed, so he's trying to sever his ties with her in the most brutal way.
She could try to talk Sasuke down. Sakura is a strong - willed character. She might start by asking Sasuke what's wrong, trying to reason with him, and reminding him of their past relationship, in an attempt to defuse the situation.
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.
It could also be related to Sakura's strength. Sasuke is constantly striving to become stronger. If Sakura somehow shows a weakness or makes a mistake during a fight that he thinks could have been avoided, and it has negative consequences for their situation, he might get angry at her for not being more capable.
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.
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 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.
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.
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 Sasuke is being influenced by some dark force. Maybe it has corrupted his mind and made him see Sakura as a threat to his new goals, whatever those might be in this fanfiction's plot. So, he uses Susanoo to hold her captive as a way to neutralize that threat.
One reason could be that Sasuke really values Sakura's friendship and is afraid of losing it to someone else. Maybe she sees others getting closer to Sakura and feels left out.