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.
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.
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.
A plotline could involve Naruto being influenced by some outside factor, like a bad rumor about Hinata, and starts to bully her. Hinata, on the other hand, might initially be very hurt but then decides to prove herself to Naruto. As time passes, Naruto discovers the truth and feels guilty for his actions. It could be a story of redemption and growth for Naruto and a story of strength for Hinata.
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 popular fanfiction might be where Hinata's bullying of Naruto is a misunderstanding. Maybe she was trying to get his attention in a really wrong way because she actually had a crush on him. For example, she could be stealing his lunch as a way to start a conversation, but Naruto takes it as bullying.
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.
One reason could be that Naruto might be too focused on his ninja goals. He has always been determined to become Hokage and might see a relationship as a distraction. In fanfiction, his single - minded pursuit of his dream could lead him to reject Hinata.