: Sasuke could be a possible cause, not out of malice but maybe during an intense fight where things get out of control. They have had some really rough battles, and in the heat of the moment, Sasuke could accidentally use a technique that affects Naruto's ability to breathe. Also, there could be some new villain in the fanfiction that we don't know about yet, who has a specific grudge against Naruto and targets his breathing as a way to weaken him.
Madara Uchiha might be the cause. Given his powerful and often devious nature, he could have devised a plan to incapacitate Naruto by interfering with his breathing. Maybe he found a weakness in Naruto's body or chakra system that allows him to create a situation where Naruto can't breathe. Another option is that it could be a side - effect of some ancient curse or seal that has been activated, and the one who set it off, whether intentionally or not, is the cause of Naruto's breathing problem.
Maybe he's in a tight situation like being trapped in a small space or under some kind of magic spell.
It could be a new villain. Maybe a villain who targets Naruto's loved ones and Naruto is unable to save them. This failure would cause him great heartbreak. For instance, if the villain kidnapped Hinata and Naruto couldn't rescue her in time.
It could be a genjutsu cast by an unknown enemy. This enemy might be targeting Naruto specifically to make him vulnerable. They could be using sleep paralysis as a way to weaken his mental state before a bigger attack.
Danzo could also be the one to make Naruto angry. Danzo's underhanded tactics and secrets might be exposed in the fanfiction, and if they were to directly harm Naruto's friends or the village, Naruto would be furious at his actions.
It could be the result of Naruto's own inner demons. Maybe he discovers some dark secret about his past, like his parents being involved in something he can't accept. This self - realization could be so traumatic that it makes him cry blood as he struggles to come to terms with it.
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.
It could be that Naruto ate a special kind of magical fruit during one of his adventures. This fruit might have given him the physiological adaptation to be able to breathe underwater, similar to how some fictional characters gain powers from special items.
Perhaps it's a result of a failed experiment by one of the village's scientists or ninjas. They could have been working on some kind of time - space ninjutsu and something went wrong. Naruto just happened to be in the wrong place at the wrong time. It might be someone like Shikamaru, who is known for his intelligence and might have been trying to create a new strategy involving time manipulation but it backfired on Naruto.
It could be Sasuke. If Sasuke completely rejects Naruto's friendship and the idea of coming back to the village, Naruto might be deeply hurt. He has always been trying to bring Sasuke back, and if Sasuke shuts him out completely, it could be a major factor in his downward spiral.