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.
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.
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.
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.
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.
Madara Uchiha could also be a likely cause. If he somehow managed to return or had a long - planned scheme that involved breaking Naruto's sanity. He was a very powerful and devious foe who could use his knowledge of the ninja world and its powers to target Naruto's mental stability.
One element that makes it funny and insane is when the author takes a normal Naruto concept and blows it way out of proportion. For instance, if the concept of chakra is turned into something like 'chakra candy' that ninjas have to eat to gain power. And when the fanfiction has a lot of physical comedy, like Naruto constantly tripping over his own feet while trying to perform a super - important jutsu. Also, when there are crossovers with really unexpected universes, like Naruto meeting the characters from a Looney Tunes world. This creates a really wacky and fun dynamic.
In this type of fanfiction, the story could also focus on how the village might react differently to Naruto. Without Kushina's presence, which could potentially have been a positive influence, Naruto might face more hostility. His struggle to fit in and gain acceptance could be more arduous, and this could be what drives him to the brink of insanity. Maybe he would develop a more jaded view of the world and the people in it.
Well, there could be a 'naruto insane naruto fanfiction' where Naruto teams up with his enemies in a very unexpected way. Maybe he forms an alliance with Orochimaru not for power, but to save the entire ninja world from a new, greater threat. They would have to put aside their differences and work together, which would be a really insane concept considering their past relationship.
In some cases, it could be a new and powerful enemy. This enemy might have the ability to manipulate chakra in a way that affects Naruto's mind. For example, they could send out waves of negative chakra that slowly erode Naruto's mental state. Over time, these attacks make Naruto go insane as he struggles to fight against this unseen and insidious force.