One possible reason could be a simple rivalry over a shared love interest. Maybe they both like Naruto, and Hinata's jealousy towards Sakura because of Naruto's attention to her could be depicted as hatred in fanfiction.
One reason could be that in some fanfiction, the characterizations of Sasuke and Sakura might be off. Maybe Sasuke is portrayed as overly cruel to Sakura in a way that doesn't seem true to his character in the original series. Another reason could be that the storylines in these fanfictions are too clichéd, like always having Sasuke constantly reject Sakura in the same uninteresting ways.
One possible reason could be jealousy. Maybe in the fanfiction, Naruto feels overshadowed by Sasuke in Sakura's eyes, which leads to his hatred. Another reason might be that he perceives their relationship as a threat to his own bonds with them. For example, he could think that Sasuke and Sakura's closeness excludes him in some way.
One reason could be creative exploration. Writers might want to see how the story would unfold if Naruto hated Hinata. It's a way to break away from the normal storyline.
In such a fanfic, one possible reason could be Naruto's feeling of being left out. Maybe he sees Sakura's constant focus on Sasuke and feels neglected in their friendship triangle. Another reason might be jealousy. If Naruto has unrequited feelings for Sakura, seeing her love for Sasuke could turn to hatred over time.
In such a fanfiction, Hinata might be jealous of Sakura because Sakura could be more popular or confident. Maybe Sakura has skills that Hinata wishes she had, like better combat skills or more outspokenness. Also, if there's a love interest involved, and the guy seems to pay more attention to Sakura, it would make Hinata jealous.
Maybe the fanfiction portrays Hinata in a way that doesn't align with her true character. For example, it might make her overly aggressive or weak in a way that doesn't fit the original concept.
In fanfiction, the evidence often lies in the way the author portrays Naruto's interactions. For example, Naruto may be shown being cold or dismissive towards Sakura, like avoiding her company or not showing any romantic interest. Meanwhile, with Hinata, he could be depicted as being more affectionate, like blushing around her or always coming to her aid. But it's important to note that this is all in the realm of fan - created interpretations and not the official canon.
Some people might write it because they prefer other characters over Naruto, Hinata, and Sakura. They could be trying to promote their favorite characters by putting these three down in their fanfiction.
One reason could be that in some fanfictions, Naruto might feel that Sakura is too obsessed with Sasuke and neglects their friendship. He values loyalty and may see her one - sided love for Sasuke as a form of betrayal to their team bond.
One possible reason could be for the sake of creating a more complex love triangle in fanfiction. Maybe the writer wants to explore a different dynamic where Naruto and Hinata's relationship is strengthened by having a common dislike for Sakura. It could also be that the fanfiction writer is trying to give Hinata more agency and make her a stronger character by having her oppose Sakura in some way, perhaps in matters related to Naruto's affections.