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 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.
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.
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.
It could be due to the failure of protecting his village or his loved ones. Naruto has always been about protecting those important to him. If he fails at that, his anger is understandable.
The fanfiction might change the important events in his life in a way that he doesn't like. For example, if it rewrites how he became a ninja or his relationship with his teacher, Iruka. These are fundamental parts of his story, and if they're distorted in the fanfiction, it's understandable that he'd get angry.
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.
One common theme is revenge. When Naruto is pissed off, often he's seeking revenge against someone who has wronged him or his friends. For example, if Orochimaru did something to harm Sakura, Naruto would be pissed off and his revenge - driven actions would be the focus. Another theme could be self - discovery. His anger might lead him to discover new powers or a new side of himself. Like his anger might unlock a hidden bloodline limit that he didn't know about before.
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.
Danzo is another suspect. He has his own dark ways and has made decisions that could easily piss Naruto off, like using underhanded methods against his friends or the village.