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.
Maybe Sakura makes a decision that endangers herself or the team without consulting Naruto. In the world of Naruto, they are a team, and Naruto values his comrades. If Sakura does something reckless like that, it could make him angry.
If Sakura betrays Naruto's trust in the fanfiction. For example, she might tell Naruto's secrets to the enemy. Naruto has always been very open and loyal to Sakura, so this would be a huge blow to him. His anger would be completely understandable as she would have broken the bond of trust between them.
Power struggle can also be a theme. If Sakura is in a situation where she has to compete with others or fight against a powerful enemy, her anger could fuel her determination to gain more power or assert her position. This could lead to intense battles or political maneuvering within the fanfiction.
Sakura could also try to stand up for herself. If she believes that Sasuke is being unfair in his anger, she may argue back. For instance, if Sasuke is angry because of a misunderstanding, Sakura might try to clarify the situation and prove her point.
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.
I'm not sure of specific sakura angry fanfiction off - hand. You could try searching on fanfiction - dedicated websites like Archive of Our Own. They have a vast collection and you can use the search filters to look for stories related to sakura being angry.
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.