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 Uraraka got involved with a villain or something shady in the fanfiction. Deku has a strong sense of justice and loyalty to his friends. If he saw Uraraka getting too close to the wrong side, whether it was because she was being manipulated or for some other reason, he would be furious. He would feel like she was betraying their shared values and the trust between them.
Well, initially it would create a lot of tension. Deku might avoid Uraraka for a while. But depending on how the fanfiction progresses, this could also be a turning point for their relationship. If they communicate about what made Deku mad, it could lead to a deeper understanding between them. For example, if Deku was mad because he was worried about Uraraka's safety, when he tells her that, it could make Uraraka realize how much he cares. On the other hand, if they don't communicate well, it could lead to a long - term rift in their relationship.
They could meet during a local hero parade. Child Uraraka is excited to see all the heroes, and Child Deku is there with his notebook, jotting down details about the heroes' costumes and quirks. They accidentally bump into each other. Uraraka apologizes and Deku is so star - struck by the idea of meeting another hero - enthusiast that he forgets to be shy for a moment. They start chatting about their favorite heroes and find out they have a lot in common.
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 this fanfiction, Uraraka and Deku might have a heart - to - heart conversation. They could share their dreams and fears, strengthening their bond as friends. Maybe they'll train together and Uraraka will improve her quirk with Deku's help.
It could be that they meet at school. Uraraka is the new girl in class and Deku is the shy but friendly boy. At first, Uraraka is a bit intimidated by the other kids' quirks, but Deku encourages her. He shows her how he's also been working hard to develop his own quirk, even though it was a late - bloomer. They bond over their insecurities and shared love for heroes.
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.
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.