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.
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.
A plot could be Naruto being tied up by a powerful new enemy who has a personal vendetta against him. This enemy might be from a clan that Naruto unknowingly wronged in the past. In the fanfiction, Naruto's friends would then have to race against time to find and rescue him. Or perhaps Naruto is tied up in a magically - enhanced trap set by a dark sorcerer who wants to use Naruto's chakra for their own evil purposes. His friends would need to figure out how to break the spell and free him.
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.
In a 'naruto and hinata grown up fanfiction', perhaps they go on adventures outside of the village. Naruto has always been curious about the world, and now with Hinata by his side, they explore different lands. Along the way, they meet new friends and foes, and Hinata's gentle yet strong nature helps them out of many difficult situations.
In such a fanfiction, perhaps the plot could revolve around Naruto and Hinata starting a family. It might show their journey through Hinata's pregnancy, with Naruto being overly protective and excited about becoming a father. They could face challenges together, like dealing with the reactions of their friends and family in the ninja village.
The main characters are Naruto and Hinata, of course. Naruto is the energetic and determined ninja who has come a long way from being an outcast. Hinata is the shy yet strong - willed kunoichi who has always had feelings for Naruto. In this fanfiction, they are the central focus as their relationship takes a new turn with the situation of Hinata being pregnant.
One possibility is that the fanfiction delves into the relationship growth of Naruto and Hinata during this special time. For example, Naruto might become more protective of Hinata. He could be shown learning about pregnancy from Kakashi or other experienced ninjas in the village. And Hinata could be preparing for motherhood by learning from her own mother's teachings or from the other women in the village.