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.
It might start with Erza being in a difficult situation where she has to choose between her own goals and Naruto. Suppose there's a powerful artifact that Naruto also needs but Erza believes it's crucial for her to obtain for the safety of her guild. In her desperation, she might end up betraying Naruto to get it. This would set off a chain of events in the fanfiction where Naruto has to deal with the emotional and practical consequences of this betrayal, and the story could then explore how he overcomes it or if he even decides to seek revenge.
Maybe in this fanfiction, there are some artifacts or powers that both Naruto and Erza are after. Erza betrays Naruto to get her hands on this power first. This could set off a chain of events where Naruto has to find a way to stop her from misusing the power. It could also make Naruto question his own judgment in trusting her in the first place.
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.
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.
Well, perhaps Erza was tricked into believing something false about Natsu. Maybe she was misled by some enemy who made it seem like Natsu was going to endanger the guild. So, in her sense of duty to protect the guild, she took actions that Natsu perceived as betrayal.
One possible reason could be a misunderstanding. Erza might misunderstand Naruto's intentions. For instance, if Naruto is on a mission that involves some secretive actions, Erza could misinterpret them as something malicious and decide to betray him.
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.
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.