In this fanfic, it could be that Sai has shown great loyalty to Naruto. Naruto values loyalty highly. While others might be caught up in their own agendas or the politics of the village, Sai might be the one who is always by Naruto's side without asking for anything in return. This makes Naruto focus more on Sai and ignore the others.
One possible reason could be that Naruto sees something in Sai that others don't. Maybe Sai has a unique way of understanding Naruto's pain or his dreams. Another reason might be that Sai represents a new beginning for Naruto. Since Sai has a different background compared to the others, Naruto might find his perspective refreshing and be more drawn to him.
Their relationship could develop in a very deep and emotional way. They might start having heart - to - heart conversations. Naruto, who usually keeps his feelings bottled up, might open up to Sai. Sai, in turn, will use his unique understanding of emotions (even if it was initially artificial but has now become real) to comfort Naruto. This will strengthen their bond, making Naruto even more attached to Sai and further ignoring the others.
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 If he was cursed, it could make him withdrawn and cause him to ignore everyone.
Perhaps Naruto has a new power that he doesn't understand and it's making him act strange. This power could be related to some ancient jutsu. As a result, he starts to ignore everyone because he's afraid of hurting them or because he's constantly in a trance-like state due to the power's influence. His friends would then try to figure out what's wrong with him and how to bring him back to normal.
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.
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.
It could be a new, mysterious ninja from a far - off land. This ninja has some sort of mind - control ability and is using it on Naruto to make him ignore everyone. The antagonist's goal might be to weaken the Hidden Leaf village by causing internal chaos through Naruto's isolation.
Perhaps Percy has received some bad news or a prophecy that makes him think that by ignoring everyone, he can protect them. So he distances himself from his friends and family, and the fanfiction could explore how they react to his sudden cold - shoulder. They might try to figure out what's wrong with him and how to bring him back to his normal self.
Perhaps Naruto is being influenced by an outside force. It could be an enemy who has tricked him into believing that his team is holding him back. Or it could be that he has fallen into a dark state of mind, like if he has some kind of mental breakdown due to all the pressure he has faced in the past. In this state, he might just push Team 7 away without really meaning to in the long run.
One possible reason could be a powerful genjutsu cast over the village. Maybe an enemy ninja with strong illusionary powers managed to make everyone forget Naruto. It could also be due to some sort of curse. For example, a dark curse was placed on Naruto that made people's memories of him fade away gradually.
One possible reason could be that Naruto has seen or experienced something that makes him distrust or fear those around him. Maybe he has discovered a dark secret about the village or his friends that has shaken his beliefs.