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.
One possible reason could be that Naruto made a comment that was disrespectful to Hinata's family or her ninja skills. For example, if he said something like her clan's techniques were weak in a thoughtless moment.
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.
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.
Hinata might be shocked at first. She has always adored Naruto, and this sudden act of aggression from him would leave her stunned and unable to process it immediately.
Well, it could be that Naruto made a really stupid mistake that hurt Hinata's feelings deeply. Maybe he accidentally said something really mean about her family or her skills in front of everyone. So in a fit of anger, Hinata slaps him to show her extreme disappointment.
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.
In certain fanfictions, Naruto might have a pre - existing relationship or unrequited love for someone else. This could make him overlook Hinata or even seem to dislike her because his heart is set on another character.