It could be due to a conflict with another team member. Suppose he had an ongoing feud with someone on the team, and it reached a point where the team decided to let him go to maintain harmony. In fanfictions, character relationships can be quite complex and often lead to such situations.
Another reason could be that he was perceived as a liability. If he was constantly getting into trouble or not performing up to the team's expectations in terms of skills or abilities, the team might have decided to kick him off. Fanfiction writers often explore such themes of a character's growth or downfall within a team dynamic.
Maybe he violated some important team rules. For example, he might have gone against the orders of the team leader during a crucial mission.
One possible reason could be a misunderstanding. Maybe there was a miscommunication during a mission, and the other members of Team Natsu thought Lucy made a big mistake. For example, if she accidentally led the enemy to their hiding place. Another reason might be jealousy. If Lucy's celestial spirit magic started to gain too much attention and overshadowed the others in some way, they could have decided to kick her out. However, this is all just speculation based on the idea of this fanfiction scenario.
One possible reason could be a misunderstanding during a mission. Maybe Erza's actions were misinterpreted by Natsu, leading to him getting hurt, either physically or emotionally.
He might be angry at first. Natsu is a passionate character, so he would likely feel betrayed and lash out immediately.
One possible reason could be a misunderstanding. Maybe someone framed Natsu for a crime he didn't commit, like stealing a powerful magical item from the guild. And the guild, without proper investigation, decided to kick him out. Another reason might be his recklessness. If his actions put the guild in great danger too many times, the guild masters could decide that he's a liability and kick him out.
Maybe he got into too many fights. Natsu has a short temper and if there were other students in the academy who provoked him or if he was standing up for someone, he might have engaged in brawls that the academy doesn't tolerate. In a place like Fairy Tail Academy, they might have strict rules about student behavior regarding fighting.
One possible reason could be that he's framed for a crime he didn't commit. Maybe someone planted false evidence to make it seem like he was endangering the guild. Another reason might be that his power got out of control and the guild, fearing for the safety of its members, made the tough decision to kick him out.
Perhaps the author wants to introduce a sense of mystery. Natsu's secret royalty could be part of a larger mystery in the fanfiction. It could be related to his past, like he was separated from his royal family as a baby for some unknown reason. Another reason might be to explore different power structures. As royalty, Natsu would have different kinds of power and influence compared to just being a dragon slayer in Fairy Tail.
Sometimes, it's just a way to shock the audience. Fanfiction authors often like to take risks and do unexpected things to their beloved characters. Torturing Natsu can be a very unexpected and extreme move that can leave a lasting impression on the readers, whether they like it or not.
It could be that Erza scolds Natsu too harshly for something he did wrong. Natsu, who might be more sensitive than he seems, could end up crying from the severity of her words.
One reason could be that the fanfictions don't match his view of the Fairy Tail world. He has his own understanding of the guild, his friends, and their adventures, and when fanfictions deviate from that, he doesn't like it.