One possible reason could be a misunderstanding. Maybe some false information is spread about Hinata that makes Naruto believe she has done something unforgivable. For example, if there's a plot to frame her for treason against the village. Another reason might be for her own safety. Naruto might think that banishing her would keep her away from a greater danger that is targeting her because of her relation to him.
Perhaps political pressure plays a role. There could be factions in the village who don't like Hinata's influence or her relationship with Naruto. They might force Naruto's hand into banishing her to maintain some sort of political balance. It could also be due to a power - hungry individual who wants to isolate Naruto and sees getting rid of Hinata as a way to do that.
Perhaps in this fanfiction, Yugito was involved in a political intrigue within the Hidden Cloud Village. Naruto, being an outsider with influence, banished her to maintain peace between different factions. Yugito's actions or associations might have been such that it was the only way Naruto saw to prevent a civil war or major conflict in the village.
One possible reason could be a huge misunderstanding. Maybe he thought Hinata did something really bad to his friends, like unknowingly leading the enemy to them. Without knowing the truth, his hot - headed nature made him lash out.
Well, one reason could be that Hinata's extreme love for Naruto gets out of hand. In the original series, she's very shy but deeply in love. In a fanfiction, this love could transform into an obsessive - like stalking behavior if the writer wants to explore a darker side of her character.
It could be due to miscommunication. Suppose there was a misunderstanding between her and Naruto. For example, Naruto might have said something that Hinata misinterpreted as him losing interest in their relationship. This confusion could have led her to seek comfort in another person, which unfortunately led to cheating.
One possible reason could be that Naruto has a dangerous mission that only he can undertake. The safety of the village or the greater good might demand his immediate departure, even though it's hard for him to leave Hinata in such a state.
One possible reason could be a misunderstanding. Maybe Hinata is misinformed about Naruto's actions or intentions. For example, she might be led to believe that Naruto has done something harmful to the village when in fact he's innocent.
One reason could be envy. Maybe Naruto sees Hinata as having something he doesn't, like a more stable family situation, and he lashes out in jealousy. Another reason might be that he's just being a typical, rambunctious kid who doesn't realize the impact of his words or actions on Hinata.
In some fanfictions, external forces might be the cause. Perhaps a powerful enemy threatens Hinata and tells her that staying with Naruto will put him in danger. So, to protect Naruto, she decides to abandon him. This adds a tragic and self - sacrificing element to the story.
One possible reason could be a misunderstanding. Maybe Naruto was in a situation where it looked like he was cheating, but in reality, it was all a big mix - up. Another reason could be that the writer of the fanfiction wants to explore a darker side of Naruto's character. Or perhaps there's some sort of outside influence, like an enemy using some kind of mind - control on Naruto to make him act out of character.
A simple reason could be that they are poisoned. An enemy might have laced their food or drink with a slow - acting poison that causes them to faint at an unexpected moment.