It could be that the fanfic writer is trying to combine the two universes in a rather dark way. In Fairy Tail, there are all sorts of magic and temptations. If a character from Naruto's world gets involved with someone from Fairy Tail, they might be easily cheated on. For example, if a Fairy Tail character has a magic ability to manipulate emotions, they could use it to deceive Naruto's character.
In some fanfictions, crossovers like this are used to show the contrast between different character traits. Naruto is from a world where friendship and hard work are highly valued. In Fairy Tail, while those values exist, there's also a lot more freedom and potential for chaos. So, if Naruto's partner in the fanfic is from Fairy Tail, they might be more likely to cheat due to the looser moral and social structures in that world.
It might be that the author is exploring a different side of the characters. Temari, being a strong - willed character, could be written into a situation where she makes a wrong choice regarding her relationship with Naruto. This could be due to her own insecurities or a desire for something else that she misinterprets as more important than her relationship at that moment.
In fanfiction, such a plot might occur due to the writer wanting to create drama. They might twist the relationships in the Naruto universe to explore extreme emotional situations. For example, the writer could be trying to show how Naruto, who is usually seen as strong and reliable, would react to such a betrayal. It's a way to take the characters out of their normal characterizations and see how they adapt.
Naruto could also experience a period of self - reflection. He might wonder what he did wrong in the relationship, even if it wasn't his fault. This could lead to him trying to improve himself in ways that are not really necessary. On the other hand, it could also fuel his determination. He may decide that he won't let this setback break him and instead use it as motivation to become stronger, both in terms of his ninja skills and his emotional resilience.
Perhaps it's a matter of a powerful enemy from Fairy Tail's lore recognizing Naruto as a significant threat and using a very underhanded or extremely powerful attack on him. Since Naruto might not be fully aware of all the rules and powers in the Fairy Tail world, he could be caught off - guard. And in fanfiction, the writer might want to create a very dramatic and unexpected death for the sake of a unique story, so they come up with a scenario where Naruto dies in the Fairy Tail world.
In fanfiction, Naruto could become the founder of Fairy Tail by having his unique ninja skills and values influence the creation of the guild. Maybe he travels to the Fairy Tail world and decides to gather like - minded mages. His leadership skills, learned from being a ninja, would help him in setting up the guild's rules and training methods.
In this kind of fanfiction, Naruto might become very distrustful. He could isolate himself from his friends for a while as he tries to deal with the pain of being cheated on. Maybe he would focus all his energy on training to forget about the heartbreak.
It could be that the fanfiction author has a different view on the characters. They might not be satisfied with the original relationship development in the Naruto series. So they come up with the idea of Hinata cheating on Naruto to show an alternative scenario. For example, they could be trying to show how Naruto would deal with betrayal, which is a very different aspect from the original story where Naruto is mainly about his growth as a ninja and his positive relationships.
Well, in fanfiction, anything can be twisted for the sake of creating drama. Maybe the author wanted to explore a really dark and unexpected scenario. The writer might have thought that by having Naruto betrayed in a Fairy Tail - related setting, it would attract readers who are into crossovers and angst. It could be due to some kind of power struggle or misinformation within the fictional world created in the fanfic.
Natsu could also be deeply hurt. He values his friends highly in Fairy Tail. If betrayed, he might isolate himself, doubting his own judgment in trusting people. This could lead to a period of self - discovery where he tries to figure out how to trust again. For example, he might recall the times when his friends in Fairy Tail were truly there for him and use those memories to heal and grow stronger in the face of this betrayal.
In such a fanfiction, the Fairy Tail guild might be thrown into chaos. They could initially be confused about this strange event from another world. Some members might see Naruto's sealing as a threat to their world's balance, while others might be more curious and try to find a way to break the seal.