Perhaps he was caught using forbidden techniques. In the Naruto universe, there are certain jutsus that are not allowed to be used casually. If Sasuke was discovered using one of these, it could lead to his expulsion. Also, if he got involved in a fight that caused extensive damage to the school property, that would be a reason for expulsion.
One possible reason could be that he was framed for a serious offense he didn't commit. Maybe someone planted false evidence of him attacking a fellow student. Another reason might be that he continuously defied the school's strict rules in order to pursue his own goals of getting stronger faster, which the school authorities couldn't tolerate.
One reason could be false accusations. Maybe someone frames Jaune for a serious offense he didn't commit, like cheating in an important exam or stealing valuable items from the school. Another reason might be his inability to keep up with the academic or combat requirements. If he fails too many classes or consistently underperforms in combat training, the school might decide to expel him.
One possible reason could be that he is wrongly accused of using dark magic. In some fanfictions, the Ministry of Magic might be more corrupt or easily influenced, and they could frame Harry for something he didn't do, leading to his expulsion.
One possible reason could be that Sasuke's past traumas and his continuous search for redemption make him feel unworthy of Sakura's love. He might think he's dragging her down and decide to end the marriage for her own good.
Perhaps Bakugou gets expelled due to his attitude problems. He might have continuously bullied a fellow student to the point where the school can't overlook it anymore. His explosive temper could lead to him saying or doing something really offensive to a teacher or a large number of students, which would be against the school's code of conduct.
One reason could be a jutsu gone wrong. Maybe an enemy uses a forbidden jutsu on Sasuke that has an unexpected side effect of changing his gender.
One possible reason could be that he accidentally causes too much destruction during a mission. Maybe he loses control of his magic and wrecks an important place in the magical world, leading the guild to expel him as a form of punishment.
In some fanfictions, it might be due to political reasons within Hogwarts. For example, if there is a power struggle between different factions, Snape could be seen as a threat or a liability to one side and they find a way to get him fired. His loyalty has always been somewhat in question in the eyes of some, so it could be exploited.
One possible reason could be that Naruto realizes Sasuke is too far gone on his dark path. After numerous attempts to bring him back, Naruto might start to think that Sasuke doesn't want to be saved. He may see that Sasuke's actions are causing too much harm to the village and the people he loves, and Naruto has to prioritize them over Sasuke. So, he gives up on the idea of bringing Sasuke back.
One reason could be academic dishonesty like cheating on exams or plagiarizing papers. Another is disciplinary issues such as fighting, bullying, or being disrespectful to teachers.
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.