Often, the theme of power corruption can be seen. Naruto might gain too much power too quickly and it starts to corrupt him, making him dark and angry. He could lash out at those he once considered friends because he no longer sees the world in the same way. His anger could be directed at the ninja world in general for its never - ending cycle of violence and hatred that he has now become a part of in a more negative way.
The other shinobi villages might also be antagonists. If Naruto's village is seen as a threat because of his power, they could target him. This would make Naruto angry as he's being attacked for no good reason other than fear. Their actions against him and his village could lead him to a darker state as he has to fight back against these external threats.
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.
Maybe Jiraiya found out some dark secret about Naruto's past, like his connection to the Nine - Tails, and decided to keep it from Naruto. When Naruto eventually discovers this, he would be angry at Jiraiya for not trusting him enough to share the truth. This could lead to a rift between them in the fanfiction.
Another reason could be in a fan - created scenario where there are secrets involved. Hinata might be keeping a secret from Naruto that, when revealed, makes Naruto feel betrayed or deceived. This sense of betrayal could easily turn into anger. For instance, if Hinata has a connection to an enemy that she's been hiding from Naruto.
Naruto being angry at Kushina in fanfiction could stem from a feeling of abandonment. Even though she's his mother, if in the fanfiction she was absent during important moments in his life, whether physically or emotionally, Naruto could be angry. He might feel that she didn't fulfill her motherly duties. For instance, if she was more focused on other things like her own past traumas or relationships, and neglected Naruto's needs, that would surely make him angry.
A typical plotline in 'angry naruto fanfiction' is Naruto's anger due to the village's past actions against him. For instance, the way he was ostracized as a child. His anger then causes him to either distance himself from the village or try to make it a better place by changing its ways. Sometimes, his anger is also sparked by threats from outside forces to the village, but the village elders don't believe in him to handle it, which makes him really angry and he takes matters into his own hands.
One possible reason could be if Sakura constantly criticizes Naruto's abilities in a very harsh way. In the story, Naruto has always been striving to prove himself, and if Sakura doesn't show any faith in him, it might make him angry.
One reason could be Boruto's recklessness. In fanfiction, Boruto might engage in dangerous stunts or actions that go against Naruto's teachings. For example, if Boruto were to use a forbidden jutsu without fully understanding the consequences, Naruto would be angry as it not only endangers Boruto but also those around him.
One possible reason could be that Tsunade makes a decision regarding the village that Naruto strongly disagrees with. For example, if she refuses to send help to a neighboring village in need, Naruto, with his strong sense of justice, might get angry. He's always been about protecting and helping others.