One possible reason could be if Katsuki is rude to Izuku. Since Inko is Izuku's mother, she would naturally get mad at someone who is mean to her son. For example, if in the fanfiction Katsuki makes fun of Izuku's dreams or abilities in a particularly cruel way in front of Inko, she would likely become angry.
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.
In some fanfiction scenarios, if Nami were to side with an enemy, even if it was for a seemingly good reason like trying to protect the crew in a way that Luffy doesn't understand. Luffy has a strong sense of loyalty to his crew and seeing Nami on the 'opposing side' might enrage him. He might feel betrayed and his anger would be a natural reaction.
In the fanfiction, if someone were to insult his dueling skills or question his abilities, that would probably make Zane mad. He takes his dueling very seriously and has a lot of pride in his skills. So any slights towards his dueling capabilities could set him off.
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 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.
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.
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.
It could be that Ron's actions were causing problems within the Order of the Phoenix. If Ron was being stubborn and not following the plans or orders set by the Order, which Sirius is a part of, Sirius might get angry. He would be frustrated with Ron for not being a team player and potentially endangering the mission and the lives of others.