One possible reason could be that she was forced to betray him. Maybe an enemy got to her family and threatened her to turn on Naruto. For example, in the Naruto universe, there are many powerful villains who would stoop to such tactics.
One possible reason could be that she is influenced by an enemy. Maybe the enemy has some sort of leverage over her or has brainwashed her into believing that Naruto is a threat. Another reason might be a misunderstanding. For example, she could misinterpret Naruto's actions as being unfaithful or harmful when in reality he has good intentions. It could also be due to her own insecurities. If she feels overshadowed by Naruto's popularity and power, she might betray him to assert her own identity.
One reason could be jealousy. Maybe Naruto's success as a ninja makes his girlfriend jealous and she betrays him out of spite. Another reason might be that she is forced to by an enemy. For example, if she is blackmailed into betraying Naruto to save her family.
It could also be due to outside influence. An enemy might deceive Naruto's team into believing that Naruto has turned against them. The enemy could plant false evidence or use mind - control techniques on the team members, causing them to turn on Naruto without realizing the truth.
One possible reason could be that she is under some sort of dark magic influence. In the wizarding world, dark magic can manipulate people's minds and actions. Maybe she was imperiused by a Death Eater or some other dark wizard who wanted to get to Harry through her. Another reason could be that she has been hiding a deep - seated jealousy. Perhaps she is jealous of Harry's fame and the attention he gets from everyone, including other girls, which could lead to her betraying him out of spite.
It could be due to a power imbalance. If Naruto's power as Hokage makes her feel overshadowed or unimportant, she might be led to betray him. Also, perhaps she discovers some long - hidden secret about Naruto that she can't forgive, like a connection to a past enemy or some dark side of his shinobi past that he has kept from her.
He might be initially shocked and heartbroken. Naruto has always been someone who values loyalty and friendship deeply, so this betrayal from his girlfriend would be a huge blow. He could go into a state of self - doubt, wondering if there was something he did wrong to make her do such a thing.
One possible reason could be fear. Naruto is extremely powerful, and some in Konoha might fear that he could one day lose control of his power. Another reason could be political intrigue. There may be certain factions within Konoha that see Naruto as a threat to their power and influence. Also, jealousy might play a part. Naruto has achieved so much and has the love of many, and some might be jealous and thus plot against him.
The story could explore how Naruto's power is affected by this betrayal. Maybe his godlike powers start to waver because his emotional state is so unstable. He might have to go on a journey to regain his confidence and control over his powers. Along the way, he could meet new characters who help him understand that not all relationships are doomed to end in betrayal. This would be a great way to develop Naruto's character further in the fanfiction.
One possible reason could be a misunderstanding. Maybe some false information was spread in the village that made Konoha believe Naruto had done something wrong. In fanfiction, things can be exaggerated and misinformation can easily lead to a situation of betrayal.
Another reason might be related to a difference in ideology. Kakashi might have a different view on how to achieve peace or protect the village compared to Naruto. This divergence could lead to actions that Naruto interprets as betrayal. For example, Kakashi might believe in a more conservative approach that involves sacrificing some of Naruto's ideals in order to maintain the stability of the village.