It could be that her plans were thwarted. If she had carefully laid out certain schemes for the betterment of her kingdom or herself, and they were disrupted, that would surely make her angry. For example, if she was trying to arrange a political alliance and it fell through due to the actions of another noble or some unforeseen circumstances.
Maybe she was betrayed by someone she trusted. In many stories, a betrayal can easily make a noble character angry.
There could be a love interest. In many web novels, a love interest can be a significant character. Maybe he is the one who can finally soothe her anger or is involved in the events that lead to her being angry in the first place.
It might be due to a thwarted plan. She could have been working on a scheme for the betterment of her kingdom or for her own power - play, and something or someone has stopped it from happening. For example, an opposing political faction has managed to block her proposed legislation. This would surely make her angry.
She might give cold glares to those who have displeased her. A single look from her can send shivers down the spines of the courtiers.
Maybe it's because of some unexpected event or conflict. Hard to say exactly without more context.
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.
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.
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 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.
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.