Perhaps in the fanfiction, Gendo has made some new demands or impositions on Shinji that are the last straw. For example, if Gendo forces Shinji into more dangerous Eva battles without any regard for his well - being, Shinji might snap in response.
One possible reason could be the long - standing emotional neglect Shinji has endured from Gendo. In the Evangelion series, Gendo is often distant and cold towards Shinji, and in fanfiction, this could build up to a point where Shinji snaps.
It could also be due to a revelation in the fanfiction. Maybe Shinji discovers some dark secret about Gendo, like his true motives behind the Eva program, and this knowledge makes him so angry that he snaps at Gendo.
It can create a major turning point. If Shinji has always been submissive before, his snapping can change the power dynamic between him and Gendo. This might lead to Shinji taking more control of his own actions and decisions in the story.
One reason could be years of built - up resentment. The villagers have treated Naruto poorly for so long, constantly shunning him and making him feel like an outcast. In fanfiction, this could reach a boiling point where he just snaps. For example, if someone in the village made a particularly cruel comment about his parents, it might be the last straw.
One reason could be stress. Izuku might be under a lot of pressure to keep up with his training and the expectations of being a hero. Class 1A's constant high - energy and sometimes chaotic behavior could push him over the edge. Another reason could be misunderstandings. If they misinterpret his actions or motives, he might snap in frustration. For example, if they think he's showing off when he's actually just trying to help.
One reason could be miscommunication. Maybe Sakura said something that Naruto misinterpreted as an insult to his dreams or abilities. For example, if she made a comment about his progress as a ninja in a way that came off as dismissive.
In fanfiction, Sasuke might snap at Sakura due to miscommunication. Sakura could be expressing her concern for him in a way that he misinterprets. For instance, if she tells him not to go on a dangerous mission and he thinks she is trying to control him instead of just caring about his safety. Another factor could be the influence of external forces. Maybe there are other characters or events that put him in a bad mood, and Sakura just happens to be there at the wrong time, so he snaps at her.
Well, one reason could be Sakura's constant nagging about Sasuke. Naruto has his own feelings and goals, and if Sakura is always pushing the Sasuke matter in a way that dismisses Naruto's feelings, it might make him snap. For example, if she keeps asking him to bring Sasuke back without considering how hard it is for Naruto.
One possible reason could be that he's under a great deal of stress. Maybe there are new threats to Berk that he's trying to deal with alone and it's getting to him. Another reason might be personal issues, like if he's having problems with his relationship with Astrid or his father. Or perhaps he's frustrated with the dragons not following his commands as he expects, which makes him lash out at everyone around him.
It could be a situation where Inari is interfering with Naruto's mission in the fanfiction. Naruto is very dedicated to his missions and protecting his friends. If Inari were to do something that put Naruto's mission or friends at risk, Naruto would likely snap at him.
It could be that he's been influenced by some sort of dark force without realizing it. In many fanfics, there are often hidden magical elements that can affect a character's behavior. So, this unseen influence might be causing him to be irritable and snap at those around him.
Another possibility is jealousy. Maybe in the fanfic, Sakura shows more interest in another character, like Sasuke perhaps, and Naruto, who has his own feelings for Sakura, snaps out of envy. It's a common trope in fan - made stories to play with the characters' emotions like this.