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.
The other ninja clans in the village might be responsible. They may feel that Naruto, with his unique power and status as the Jinchuriki of the Nine - Tails, could disrupt the balance of power among the clans. So they work together to ensure he loses his birthright.
In such a fanfiction, perhaps Naruto could be wrongly accused of something that leads to him being stripped of his birthright. Maybe a false claim about his connection to the Nine - Tails is used against him.
Another possibility is that they were deceived. There could be a villain who tricked them into believing that giving away Naruto's birthright was the right thing to do. They might have been misinformed about the consequences and the true nature of what they were giving up.
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.
Iruka - sensei could be one of those who help Naruto. He has always been like a father figure to Naruto and would offer emotional support. Rock Lee could also be a great helper. Lee, who has trained his body to be extremely powerful, could teach Naruto some physical combat techniques that don't rely on eyesight. And of course, the Nine - Tails inside Naruto might also play a role. It could potentially share some of its power or knowledge to help Naruto deal with his new situation.
This change in the fanfic would likely have a huge impact on Naruto's character. Without his birthright, he may be more distrustful of the system that allowed such a thing to happen. His usual optimism might be tinged with cynicism. For example, he might be less likely to believe in the Hokage system if it was involved in taking away his birthright. He'd have to re - evaluate his goals and dreams, and this would be a major part of his new character development.
One possible reason could be for Naruto's protection. His parents might think that by giving away his birthright, they can keep him away from dangerous enemies or political intrigue in the ninja world. Maybe they foresee a great threat that would target Naruto specifically because of his birthright, and this is their extreme measure to ensure his survival.
Perhaps he was in a battle against a very strong enemy. The enemy might have used a special attack that specifically targeted his eyes, like a genjutsu that damaged his optic nerves when he broke free from it.
Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.
He might be initially very confused and hurt. Naruto has always longed for a connection to his parents, and finding out they gave away his birthright could make him feel like he was unwanted or that they didn't trust him with it. He could go through a period of self - doubt, wondering if he is really worthy of being their son.