One reason could be Harry's resemblance to his father. Snape had a long - standing grudge against James Potter, and seeing Harry might trigger those old feelings. In slash fanfiction, this can be exaggerated, with Snape being harsher on Harry because of his connection to James.
Harry's often - shown disregard for Snape's authority in the classroom. He might talk back or not follow instructions properly. This would surely annoy Snape, who is strict about discipline in his class, and thus lead to punishment in the fanfiction world.
In some slash fanfiction, there could be a plot - driven reason. For instance, if the author wants to create a tense relationship between Snape and Harry to later develop a more complex relationship (not necessarily romantic in a traditional sense but a deep - seated connection). The initial punishments serve as a way to build that tension and animosity between them. It can also be a way to show the power dynamics at play in the Hogwarts environment, with Snape representing the strict teacher and Harry the rebellious student.
It could be that in the context of the fanfiction, Snape believes Harry has been disrespectful towards him or other teachers. For example, if Harry were to talk back or show disobedience in class, Snape would likely punish him as a way to assert his authority.
Another reason might be that Harry is suspected of sneaking around in areas where he's not supposed to be. Snape, as a vigilant teacher, may punish Harry to maintain discipline in the school. He may think Harry is up to something related to the ongoing dangerous situations in the wizarding world and wants to keep him in line.
In some fanfics, Snape might punish Harry because of his perceived insolence in class. Harry has a rebellious streak, and Snape, being strict, wouldn't tolerate any behavior that he deems disrespectful to his authority as a teacher.
In some fanfictions, Snape might punish Harry by giving him extra detention tasks like scrubbing the cauldrons in the Potions classroom until they shine. This is a common form of punishment as it's within Snape's domain as the Potions master.
One reason could be Draco's arrogance. If Draco shows too much pride and disrespect in class, Snape, who values discipline, would not tolerate it. For example, if Draco mocks other students' potions attempts, Snape might punish him.
One possible reason could be that Snape fails to deliver crucial information. Voldemort is a very demanding and unforgiving master. If Snape doesn't give him accurate details about the Order of the Phoenix's plans, for example, Voldemort would surely be enraged and punish him.
In fanfiction, Snape might punish Draco for getting involved with the wrong crowd. For example, if Draco starts to associate with Death Eaters - in - training or those with malicious intentions. Also, if Draco fails to complete his academic tasks or cheats in exams, Snape would surely take action. This is because Snape values knowledge and academic integrity in Hogwarts. Additionally, if Draco causes trouble during important school events or ceremonies, Snape would likely punish him to set an example for other students.
One possible reason could be if Harry were to break a major school rule in the fanfic, like using dark magic without proper authorization. Dumbledore is a strict adherent to the rules of Hogwarts and would likely punish such behavior.
One possible reason could be Snape's long - standing grudge against James Potter, Harry's father. Since Snape had a difficult past with James, he might project that onto Harry. Another reason could be that Snape is trying to protect Harry in a very strange way. He may think that by rejecting Harry, he is keeping him at a distance from the dangerous wizarding world politics and plots that Snape is often involved in.
Snape could be distracted by his own dark thoughts or memories. For example, if something reminded him of his past with James Potter, his hatred for Harry's father might cloud his judgment for a moment and cause him to accidentally cast a spell in Harry's direction.