Well, it's not that common, but it does pop up. I think it may exist because in the Harry Potter world, Snape has this brooding and often unlikable exterior. Writers may use the idea of him getting spanked as a way to kind of 'put him in his place' in a very extreme and fictional sense. It could also be a way to show his relationship with authority figures in a more extreme and unique way.
One interpretation could be that it's a form of discipline for his actions in the wizarding world. For example, if we consider his actions towards Harry Potter and other students, some fanfiction writers might use this as a way to show him facing consequences. Another interpretation could be a more comical or satirical take on his serious and often intimidating character, like a way to break down his stern facade.
In fanfiction, Snape might be drawn to Hermione because of her intelligence. She's extremely bright and her thirst for knowledge could be appealing to Snape, who also values intelligence highly. Her ability to think quickly and solve problems might catch his attention.
It's inappropriate because it involves a rather violent and unethical scenario within the context of the 'Harry Potter' characters. Hermione is a strong, intelligent character, and Snape is a complex teacher figure. This kind of relationship in the form of such an act is not in line with the original characterizations and the overall positive and magical atmosphere of the series.
One interesting fanfiction could be 'A Different Perspective'. In this, Snape starts to notice Hermione's intelligence in a new way during a special project in Potions. It shows their relationship evolving as Snape realizes Hermione's potential beyond what he initially thought.
Writers may also use this theme as a way to explore the darker aspects of the wizarding world that aren't always shown in the original books. The self - harm could be a symbol of how the society in the wizarding world failed certain individuals, like Snape. He was constantly under pressure and ostracized in some ways. This theme can be a vehicle to discuss mental health in a world full of magic but also full of real - life - like problems.
Well, in the Potterverse, Snape is a very polarizing character. Some fans love him, and some don't. Those who don't might be more likely to write him as a bully in fanfiction. It could also be that they want to create an 'underdog' situation for Harry. By having Snape bully him, Harry becomes more of a hero when he stands up to it or finds a way to deal with it gracefully.
Yes, it is a somewhat common theme in some fanfictions. One reason could be the complex relationship between Harry and Snape in the original series. Snape was often cruel to Harry, so in some alternate universes, writers explore the extreme scenario of Harry killing Snape. It also allows for exploring darker aspects of Harry's character and the consequences of such an act within the wizarding world.
The Death Eaters are a likely choice. Since Snape was a double agent, some of them might have suspected his true loyalties all along. Once they got the chance, they would kidnap him to either punish him or to get information out of him about the Order of the Phoenix.
A possible plotline could be that Severus was framed as a Death Eater. He might be trying to clear his name throughout the fanfiction. Another idea is that he is part of a secret organization that is dedicated to taking down the Death Eaters from the inside, but they have to operate in secrecy so it appears he is one of them.
In many stories, disrespect is also a cause. Saying rude things to mom or not listening when she's talking. Moms spank to teach kids to respect others, especially family members. It's their way of instilling good values in a rather tough but sometimes necessary way.