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.
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 during a mission. Maybe Erza's actions were misinterpreted by Natsu, leading to him getting hurt, either physically or emotionally.
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.
It could be due to a series of bad decisions that made him seem like a bad influence in Gryffindor. For example, if he constantly got into fights with other Gryffindors and disrupted the harmony of the house, they might kick him out. Also, if he was caught breaking too many school rules in a way that reflected poorly on Gryffindor as a whole, like sneaking out at night to go to the Forbidden Forest too often without a good reason, that could lead to his expulsion.
It could also be due to a major disagreement with Kakashi. Maybe Naruto has a different vision for how they should operate as a team or a particular mission that goes against his values, causing him to leave.
Maybe in the fanfiction, Marshall made a big mistake that endangered the other Paw Patrol members. For example, he might have accidentally set off a chain of disasters during a rescue mission due to his over - enthusiasm and lack of caution.
One possible reason could be his unruly behavior. Naruto is known to be a bit of a troublemaker in the series, always pulling pranks. In fanfiction, this might be exaggerated, and the orphanage might not be able to handle it anymore. Another reason could be that he's suspected of being involved in something dangerous or illegal, even if he's innocent. For example, if there are strange incidents around the orphanage and Naruto is the easy target to blame because of his reputation.
One possible reason could be his prankster nature. Maybe he played one too many harmless but annoying pranks on the staff or other orphans, and they decided they'd had enough. Another reason might be the fear of the Nine - Tails within him. Even if the people in the orphanage didn't fully understand what it was, they could sense something dangerous about Naruto and wanted to distance themselves from him for safety reasons.
Perhaps Naruto's uncontrollable use of the Nine - Tails' chakra endangers the team during a mission. The village leaders see this as a huge risk and order him to be removed from Team 7. Naruto then embarks on a journey to better control his chakra on his own. He might meet some new friends or mentors along the way who help him become stronger and more in control, and eventually, he tries to make his way back to Team 7.