Perhaps there is some sort of misunderstanding between them. Nero might have misinterpreted something Trish said or did, and in his reaction, he does something that offends her. In fanfiction, miscommunication can often lead to one character getting angry at another.
It could be that Nero is being overly reckless in the fanfiction. His actions might put himself or others in danger, and Trish, being more cautious perhaps, gets pissed off at his lack of regard for safety.
Maybe in the fanfiction, Nero has made some decisions that go against Trish's beliefs or values. For example, if Nero decides to trust a character that Trish knows is untrustworthy, it could make her angry.
It might become strained. They could start avoiding each other for a while until they resolve the issue. In some cases, Trish might be less likely to help Nero in his quests or battles if she's really pissed off.
One possible reason could be Nero's over - eagerness or recklessness. Maybe Nero keeps getting into dangerous situations without proper planning, and Dante, being more experienced, gets pissed off at Nero's lack of caution.
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.
In some cases, Dante could distance himself from Nero for a while. He might go off on his own to cool down and think things through. Meanwhile, Nero could be left feeling guilty or confused about why Dante got so angry.
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.
Another reason might be related to his family. In some fanfictions, his parents' story or his relationship with his family members could be distorted. Harry has a strong connection to his family, especially his parents, and any wrong portrayal of that could make him pissed off.
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.
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.
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.
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.