It could be due to a misunderstanding. Perhaps someone spread false rumors about him, and as a result, people started treating him differently. He might be angry at everyone because he feels that no one bothered to find out the truth from him. Also, if there were some new rules or restrictions imposed on him without proper explanation, that could fuel his anger.
His family might have neglected him in some ways. Naruto grew up alone and feeling unwanted in the original story. If in the fanfiction, his family was around but still didn't give him the love and attention he needed, it would be a valid reason for his anger. He has always craved connection and acceptance, and if his family failed to provide that, he would be furious.
One possible reason could be that Harry made a dangerous decision without consulting Sirius. For example, if Harry decided to face a powerful dark wizard alone, Sirius, being his godfather and concerned for his safety, would be angry.
One possible reason could be that Harry has been spending too much time on his heroic duties and not enough time with Ginny. Another might be that he made a decision without consulting her, like taking on a particularly dangerous mission. It could also be something as simple as he forgot an important date or anniversary.
One common reason is Hermione's over - protectiveness. Sometimes she tries to control situations too much, which can make Harry feel suffocated and thus angry. For example, in some fanfictions, she might try to stop Harry from taking risks in a dangerous mission, which he sees as her not believing in his abilities.
It could lead to a lot of conflict. His friends might try to reason with him, but if he's too angry, he might push them away. This could split their group apart, creating new alliances or just leaving Harry isolated for a while. His anger can also attract the attention of the wrong sort of people, like those who want to use his emotional state for their own gain.
One possible reason could be that the Cullens make a decision about her future without consulting her. For example, if they decide she should leave Forks for her own safety without asking her how she feels about it.
Perhaps he discovers some secrets about his parents that he doesn't like. For example, he might find out that they were involved in some dangerous or unethical magic before. This new knowledge could make him angry at them for putting themselves and their family in jeopardy.
He might lash out with his magic. For example, he could send a spell flying at the source of his anger without really thinking. Sometimes, he just glares intensely, his green eyes filled with fury. And he may raise his voice, shouting at those who have angered him.
One possible reason could be that Erza might have taken on a dangerous mission alone without informing Natsu. Natsu is protective of his friends and would get angry if he thought Erza was putting herself in unnecessary danger.
Perhaps Naruto has always felt different from others in the village. And when he finally learns about his parents, he realizes that their actions, even if they were for a good cause like protecting the village, still made his life very difficult. He might be angry that they didn't find a better way to handle things so that he didn't have to suffer so much growing up.