In some fanfiction, this could happen due to a misunderstanding. Maybe Hinata is forced into a situation where she has to seem like she's betraying Naruto. For example, she could be blackmailed by an enemy who threatens the safety of the village or her family. Another possibility is that there is some sort of mind - control involved that makes her act against Naruto.
One reason could be a miscommunication. Suppose Hinata gets false information about Naruto doing something bad. Without proper investigation, she might believe it and take actions that seem like a betrayal. Also, in the world of fanfiction, writers often like to take extreme creative liberties. They might want to create a completely new and unexpected storyline where Hinata, who is typically seen as a loyal character, turns against Naruto for the sake of creating drama and new plot twists.
Hinata's betrayal could be a part of a bigger conspiracy. Maybe there are other forces at work in the ninja world that want to bring down Naruto. They use Hinata as a pawn. As the story unfolds, Naruto has to not only deal with Hinata's betrayal but also uncover the conspiracy and save the ninja world from a greater threat.
Well, in fanfiction, the author might create this situation to explore new storylines. Maybe the author wants to show Naruto's vulnerability. Erza could be made to betray Naruto due to some misinformation or a dark influence. For example, if there's an evil force that manipulates Erza into believing Naruto is a threat to her world or friends.
In this kind of fanfiction, it might happen because of a misunderstanding. Maybe Rias is misled by false information about Naruto. For example, some villains could fabricate lies about Naruto's intentions or actions, causing Rias to believe he's a threat and thus betray him.
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.
Another aspect could be influenced by external forces. Let's say an evil organization manipulates Naruto's family. They use mind - control or blackmail to make his family act against Naruto. In fanfiction, this can add a layer of drama and tragedy. It makes Naruto's journey even more difficult as he has to not only face external enemies but also deal with the betrayal of his own family.
It could also be a misunderstanding. Hinata might be under some sort of mind - control or false impression. For example, an illusion is cast on her that makes her believe Naruto has become evil. As a result, she takes actions against him that he perceives as betrayal. In this case, once the truth is discovered, they would likely reconcile.
In such a fanfiction, it could be due to a misunderstanding. Maybe Naruto was forced into a situation where he had to act against the Yugioh world for the sake of his own village. For example, if there was a threat to Konoha that was somehow related to the Yugioh realm and he was given no other choice but to 'betray' them.
Another possibility is a misunderstanding. Maybe some false information is spread about Naruto that makes Tsunade believe he has gone bad. In the world of Naruto, there are often enemies who try to create chaos by sowing seeds of distrust. Tsunade, without fully investigating, could fall for such a trick and seem to betray Naruto.
Another possibility is a complete character distortion in the fanfiction. The writer might decide to create a 'bad Naruto' version where he has lost all his positive traits. However, this is quite far - fetched considering the strong foundation of Naruto's character in the original series. It would be a very different and probably controversial take on the Naruto - Hinata relationship.
Well, one possible reason could be a conflict of ideals. Naruto, with his strong belief in friendship and protecting the village, might find himself at odds with the Bleach universe's complex system of souls and Hollows. Maybe in this fanfiction, Naruto is somehow forced into a situation where he has to choose between his own world and the Bleach world, and his choice is seen as a betrayal by the Bleach characters.