Another possible reason is that Naruto's goals and dreams as a ninja become so all - encompassing that he simply doesn't have time for Sakura. His pursuit of power, knowledge, or the protection of the village takes precedence. He may see Sakura as a distraction from these more important matters and thus starts to ignore her.
In some fanfics, Naruto may ignore Sakura because he has his own emotional traumas to deal with. He might find Sakura's presence a reminder of things he wants to forget or move past, so he chooses to keep his distance. For example, if he has faced a lot of rejection from her in the past, he could decide to just cut her out of his life.
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.
Another reason could be that Naruto is trying to protect Sakura in his own way. He might think that by ignoring her, he can keep her out of harm's way. If he senses that there are dangerous enemies lurking around, he could be trying to create a distance between them so that Sakura doesn't get caught up in any potential battles or threats.
One possible reason could be that Naruto has his own goals and missions that he is so focused on that he tries to avoid distractions, and Sakura might be seen as a distraction in some sense. For example, he is always aiming to become Hokage and improve his ninja skills, and any personal relationship drama could get in the way of that.
It could also be that Naruto is forced to leave Sakura due to some external threat or danger. For instance, if there is an enemy targeting Naruto and he fears that being around Sakura will put her in harm's way, he would choose to leave to keep her safe. This is in line with Naruto's selfless nature as seen throughout the series.
Sakura might initially be confused. She would probably wonder why Naruto is acting this way all of a sudden. For example, she might try to get his attention by calling his name a few times, but when he doesn't respond, she could be left feeling a bit bewildered.
Maybe he got some kind of curse. In the Naruto universe, there are all sorts of dark magic and curses. If he was cursed, it could make him withdrawn and cause him to ignore everyone.
One possible reason could be that something happened during the Kage Summit that made Naruto lose trust in Sakura. Maybe she made a decision or took an action that he didn't agree with. For example, if she sided with someone against Naruto's ideas regarding the situation at the Summit.
One possible reason could be that he has some unresolved issues from his past, like the absence of his parents during his childhood which made him feel a sense of detachment. Maybe he is so focused on achieving his ninja goals that he unconsciously starts to ignore his family. Another reason might be some sort of miscommunication within the family. For example, if there are secrets that the family has been keeping from him, he might feel betrayed and thus choose to ignore them.
Another possibility is jealousy. Maybe in the fanfic, Sakura shows more interest in another character, like Sasuke perhaps, and Naruto, who has his own feelings for Sakura, snaps out of envy. It's a common trope in fan - made stories to play with the characters' emotions like this.
Another reason could be a very dark and alternative interpretation of the characters. Maybe the writer has a different view of Naruto's character arc and how he could be pushed to the extreme. And they see Sakura as a character that could be sacrificed in this extreme version of Naruto's story. But this is a very far - fetched view considering the original story's development.