Another factor could be some sort of external influence, like a new, powerful and malevolent enemy who has the ability to manipulate his mind. This enemy could use his past traumas against him, causing him to lose his grip on reality. For example, they could make him relive the moments when he almost lost Pepper or his friends in a loop, driving him insane.
One possible cause could be the extreme stress he endures. Constant battles, the fear of failing to protect the world, and the loss of friends can all take a toll on his mental state. In fanfiction, writers might explore how the weight of these experiences gradually pushes him over the edge.
Perhaps in some fanfiction, an experiment gone wrong might be the cause. Tony Stark is always tinkering with new technology. If he were to test a new piece of equipment that had unforeseen side effects on his brain chemistry, it could lead to his insanity. This could be something as simple as a new type of arc reactor that emits waves that interfere with his neural functions.
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.
Captain America would likely try to be the voice of reason. He'd approach Tony with empathy, trying to understand what's going on in his head and find a way to bring him back to his senses. He might also try to keep the other Avengers calm and focused on helping Tony rather than seeing him as a threat immediately.
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 possibility could be that after facing too many dark magic encounters, Harry's mind starts to break. Maybe the constant threat from Voldemort and the losses he endures, like the death of his friends, push him over the edge. He could start to have hallucinations and lose his ability to distinguish between reality and dark magic-induced illusions.
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.
Maybe it could be due to the loss of his friends. For example, if Sasuke died in a battle and Naruto couldn't save him, the grief might push him over the edge and into insanity.
The main antagonist could be Naruto himself in a sense. Since he has gone insane, his actions are now unpredictable and harmful to the people around him. His former friends, like Sakura or Kakashi, might have to step up and try to stop him, seeing him as the new threat.
One possible plot could be that Naruto is pushed over the edge by the constant betrayal he faces. For example, if the village he has always protected turns against him due to false accusations of being a threat. He might start using his powers in a more erratic and dangerous way, not caring about the consequences. This could lead to a showdown between him and his former friends who now see him as a menace.
It could be a new dark wizard who has the power to manipulate minds. This wizard might target Harry specifically, driving him insane as part of a larger plan to take over the wizarding world. Since Harry is seen as a hero, if he goes insane, it would create chaos and make it easier for the antagonist to achieve his goals.
Stress could be a factor. If Iron Man is under a great deal of stress, he might overeat. For example, if he's facing a new, extremely powerful enemy that he's not sure he can defeat, he could turn to food for comfort. Another reason could be an injury. If he's injured and unable to exercise as he normally does, his metabolism might slow down and he could gain weight.