Sasuke could be the cause. If Sasuke were to betray Naruto again in the fanfiction, it would surely make Naruto pissed off. Their relationship is complex and Sasuke's actions have a big impact on Naruto.
A new, powerful and arrogant villain introduced in the fanfic could be the cause. This villain could constantly mock Naruto and his ideals, and try to undermine his efforts to protect the village. Such blatant disrespect would make Naruto really pissed off as he is very protective of his home and the values he stands for.
One possible scenario could be that Naruto gets pissed off when he discovers someone has been spreading false rumors about his friends in the fanfiction. This would really push his buttons as he values his friends highly.
Sasuke could be a possible cause. If Sasuke were to go back on his word about protecting the village or if he were to harm their friends again, Naruto would be pissed. Their relationship is complex, and Sasuke's actions often have a strong impact on Naruto.
Sasuke could make Naruto pissed off if he goes back to his old ways of seeking power and revenge. Naruto has worked hard to bring Sasuke back to the right path, and if Sasuke strays again, it would surely anger Naruto.
In a 'naruto gets pissed off fanfic', one possible scenario could be that Naruto discovers someone has been slandering his friends behind their backs. He would get pissed off and confront the person directly, his eyes blazing with anger.
A sense of powerlessness can also be a theme. When he's faced with situations where he can't immediately solve a problem or protect those he cares about, Harry might get pissed off. This often leads to him finding new ways to gain power or take control in the fanfiction.
One reason could be betrayal. If someone he trusted, like a fellow ninja or an old acquaintance, turned out to be working for his enemies, that would surely make him pissed. It's a huge blow to his trust, which is a big part of his character.
One scenario could be when Naruto finds out that someone has been spreading false rumors about his friends. This would surely make him pissed as he values his friends deeply.
Another good one is 'When Harry's Temper Flared'. Here, it starts with a small incident in the Gryffindor common room that snowballs into a major event where Harry gets extremely pissed off. The writer focuses on Harry's relationships with his friends during this time of anger and how they react to his outbursts.