It could be the Hidden Leaf Village's council. If they were to unjustly accuse Naruto of a serious crime he didn't commit, like being a traitor, and try to imprison or execute him, that would surely make him lose it. He has always strived to protect the village, and such a betrayal from its leaders would be a huge blow.
It could be Orochimaru. He has always been a source of trouble and if he manages to pull off some really devious plan that directly affects Naruto or his loved ones, it might cause Naruto to snap.
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.
There's also the possibility that it's an accidental injury caused by one of Naruto's friends during a training session gone wrong. For example, Sasuke could accidentally unleash a jutsu that hits Naruto's eye while they are sparring.
It could be Sasuke. If Sasuke has some sort of newfound connection with Naruto and it causes Naruto to shift his focus away from Sakura, then Sasuke would be the cause. For example, if Sasuke and Naruto team up for a powerful mission and their bond deepens in a way that makes Naruto neglect his relationship with Sakura.
Danzo is another suspect. He has his own dark ways and has made decisions that could easily piss Naruto off, like using underhanded methods against his friends or the village.
Arthur could be the cause. He often bosses Merlin around and doesn't fully understand Merlin's capabilities. His constant demands and lack of respect for Merlin's intelligence might push Merlin to the edge. For instance, when Arthur makes Merlin do dangerous and pointless tasks without any regard for his safety.
Sasuke could be a possible cause. If Sasuke were to go back on his word about protecting the village or if he were to harm their friends again, Naruto would be pissed. Their relationship is complex, and Sasuke's actions often have a strong impact on Naruto.
A new, unknown villain could be the cause. This villain might have a special power related to mind control or mental manipulation. They target Naruto because of his status as a hero in the ninja world. By making Naruto lose his mind, they could throw the entire ninja world into chaos.
It could be Orochimaru. He is known for his devious ways and might manipulate Naruto into believing false things about the ninja world.
It could be Danzo. He was always power - hungry and had his own ideas about what was best for the village. He might see Naruto as a threat and find a way to take away his birthright.