In some fanfictions, Sakura may make decisions without considering Naruto's opinion. Naruto values his friends and their input, so if Sakura goes ahead and does something like endangering the team or making a big decision on her own that affects everyone, Naruto could get mad. Also, if Sakura were to insult Naruto's dream of becoming Hokage, that would definitely be a reason for his anger as that dream is very important to him.
One possible reason could be if Naruto in the fanfiction is being unfaithful to her. In the original series, their relationship is built on trust and love. So, if the fanfic portrays Naruto in a way that goes against that, Hinata might get mad.
Well, it might be that the fanfiction focuses too much on Sakura in a way that Naruto feels is unfair to the other characters. Naruto is part of a big world with many important people, and if the fanfic just hones in on Sakura all the time, he could get fed up. Also, if the fanfiction has Sakura constantly putting Naruto down or not appreciating his efforts like in the real story, that could be a reason.
Maybe Chi - chi is constantly comparing Gohan to Goku in the fanfiction. Gohan has his own identity and abilities, and being compared all the time can be frustrating. He could get mad because he wants to be recognized for who he is, not just as Goku's son. Also, if Chi - chi is interfering too much in his training or friendships, that could also lead to his anger.
Perhaps Percy forgets an important promise he made to Annabeth. In their relationship, promises are significant. If he fails to keep one, like a promise to help her with a project or be at a certain place at a certain time, Annabeth would surely be mad. It could also be that Percy accidentally breaks one of Annabeth's creations or possessions, which she holds dear.
Maybe Tails was tinkering with Sonic's shoes or some other personal item without permission. Sonic values his stuff and his independence. If Tails was messing around with his things in an attempt to 'improve' them but instead caused a problem, Sonic would likely be angry.
One possible reason could be unrequited love. If in the fanfiction, Sakura continuously rejects Naruto or shows no interest in him, it can lead to his depression. Naruto has deep feelings for Sakura for a long time, and constant rejection can be soul - crushing.
Perhaps in the fanfiction, there is a plot twist where Sakura has been brainwashed by an antagonist. The antagonist could be using her to get to Naruto, knowing their close relationship. So, she would then be made to attack Naruto without her own will. Or, there could be a prophecy or some sort of pre - determined fate in the fanfiction's plot that causes Sakura to hurt Naruto, even if she doesn't really want to.
One possible reason could be the stress from her ninja life. Constant battles, the pressure to protect the village, and the fear of losing her loved ones can take a toll on her mental health. Another reason might be her relationship issues, especially her unrequited love for Sasuke which has been a long - standing source of emotional turmoil for her.
In the fanfiction, Beast Boy could have eaten all of Raven's favorite snacks without asking. This simple act of thoughtlessness might set her off. Or he could have interrupted her meditation multiple times, which is very important to her. Also, if he was being too over - confident in a situation where Raven knew they needed a more careful approach, his cockiness could be a reason for her anger.
Maybe in the fanfiction, Uraraka said something that hurt Deku's feelings deeply. She could have made a comment about his abilities or his dreams in a negative way, which would definitely make Deku mad. Deku is sensitive about his role as a hero - in - training and his goal to save people, so any negative remarks about that from someone he cares about like Uraraka could set him off.