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.
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.
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 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.
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.
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.
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.
One reason could be their long - standing feelings for Naruto. They both care about him deeply, and in fanfiction, this can lead to intense rivalry. Sakura may feel that she has known Naruto longer and has a stronger claim. Hinata, on the other hand, has her own deep, unspoken love for Naruto that she might be more willing to fight for. Another reason could be their own insecurities. Sakura might be insecure about Hinata's gentle and kind nature that Naruto seems to like. Hinata could be insecure about Sakura's strength and her history with Naruto.
In the fanfiction, Sakura could be jealous because she has an ego. She might have thought that Naruto would always be there waiting for her. But when he falls for Hinata, it bruises her ego. Additionally, she may have seen Naruto as her safety net, someone who would always support her no matter what. Losing that special status to Hinata makes her envious and jealous.