Well, it really depends on the writer's preference in the fanfiction world. If the writer wants to focus on the initial crush Naruto had on Sakura, they might make Naruto and Sakura get together. However, if they are more into the idea of Naruto finally realizing Hinata's true feelings and reciprocating, then Naruto will be with Hinata.
It could be for the sake of creating an interesting and unique plot. Writers like to take characters out of their normal situations to explore different aspects of their personalities and relationships.
Hinata and Kageyama don't have a romantic ending in the manga. The story emphasizes their volleyball journey and the development of their skills and teamwork, not a romantic connection.
One popular fanfiction might be on platforms like FanFiction.net or Archive of Our Own. You can search using the keywords 'hinata tied up naruto' on these sites. However, make sure to follow the rules and regulations of the platforms.
Naruto could be initially in shock. Given his strong feelings for Hinata, he might not believe it at first. He could try to talk to her and ask for reasons, hoping to fix things.
One possible reason could be miscommunication. Maybe Hinata felt that Naruto was always so focused on his ninja duties and didn't have enough time for her. Another reason might be external pressure. For example, if there were some political or family issues in the Hidden Leaf Village that made their relationship difficult to maintain. It could also be that Hinata's own insecurities got the best of her. She might have felt that she wasn't good enough for Naruto, who was a hero in the village.