It could be that Naruto discovers some dark secrets about his past, such as the real reason his parents died or some hidden and malicious intentions of the village elders. This overwhelming knowledge could be too much for him to bear and push him over the edge into insanity. He would then start to act erratically, causing turmoil in the ninja world.
A plotline could be that Naruto is exposed to a powerful genjutsu that makes him lose his grip on reality. As he tries to fight it, he gradually goes insane. He could start seeing hallucinations of his worst nightmares, like the death of all his friends. This could cause him to lash out at those around him, thinking they are part of the genjutsu, and his actions would be very different from his normal self.
It could be Sasuke. If Sasuke decides to fully embrace the dark side and wants to bring chaos, he might target Naruto specifically. His actions and taunts could be what drives Naruto insane.
It could also mean that Naruto's actions, which seem normal on the surface, are actually part of a more complex and insane plan in the context of the fanfiction. For example, his pursuit of becoming Hokage might have a much darker and more insane motivation that he keeps secret.
Perhaps in the fanfiction, some sort of dark magic or curse has been placed on Naruto. This could be messing with his mind and causing him to act in an insane manner. It could be a way for the author to introduce a new and dark plot element to the story. Another cause might be that he has been betrayed by those he trusts the most, which shatters his mental state.
Obviously Naruto himself is the main character as it's about him being driven insane. But often, Sasuke can also be involved as his actions or relationship with Naruto can be a trigger for Naruto's insanity.
One possible plot could be that the Kyuubi's chakra constantly overwhelms Naruto's mind, leading him to make rash decisions in battles. For example, he might attack his allies by mistake when the insanity takes over.
A plot could involve Naruto being cursed with some kind of insanity - causing him to act erratically. Hinata, being in love with him, refuses to give up on him. She tries to find a cure while also dealing with the changed Naruto. Their relationship could become strained as Naruto struggles with his inner turmoil and Hinata tries to hold on to the Naruto she knew. But in the end, through their love and Hinata's determination, Naruto overcomes the insanity.
Well, one scenario could be that Kushina finds a fanfiction where Naruto has turned evil due to some dark force. Another might be that she discovers a story where Naruto is in a relationship that is completely out of character for him. Maybe she stumbles upon a fanfiction where Naruto has some really strange and overpowered abilities that don't fit the original Naruto universe.
One possible plot could be that Naruto is constantly ostracized by the villagers even after all he has done for them. The years of being treated as an outcast and having his good intentions misconstrued finally push him over the edge. He starts to see the villagers as the real enemy and decides to use his power not for their protection but to make them feel the pain he has endured.
A possible plot could be that Vash's world and Naruto's world start to merge. Naruto meets Vash during this chaotic time. Vash is initially skeptical of Naruto's ninja ways which seem so violent compared to his own. However, as they work together to save both of their worlds from destruction, they come to understand and respect each other. Naruto, inspired by Vash's selfless nature, starts to change his approach to fighting, and this causes a stir in his own ninja community as they are not used to such a pacifist - like Naruto.