The Akatsuki as a whole could be the opponents. Imagine Naruto facing multiple Akatsuki members at once in a fight. The combined threat of their different powers and their goals of capturing the tailed beasts could be overwhelming for Naruto. They might use underhanded tactics, like threatening to kill captives or using mind - control techniques on Naruto's friends. All of these things could make Naruto go insane as he battles against them, trying to protect what he loves.
Well, there are several factors. Firstly, a monotonous environment with no variety or novelty can do it. Secondly, if she has no goals or passions to pursue, it can lead to boredom-induced insanity. Also, a lack of intellectual stimulation might play a role.
Well, a 'fanfic' like this might explore new and unique storylines within the Harry Potter universe. It could potentially have characters facing death or the concept of death in a different way, inspired by 'Don't Fear the Reaper'. Maybe it shows how Harry and his friends deal with the ever - present danger in their world in a more philosophical or even fearless manner.
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.