One possible reason could be that Hermione's behavior might have put herself or others in danger. For example, if she was being overly reckless in her pursuit of knowledge, like delving into dark magic without proper precautions. Sirius, having experienced a lot in the wizarding world, would be worried about her safety.
It could be that Hermione's actions went against some unwritten wizarding code or tradition that Sirius holds dear. Since Sirius comes from an old wizarding family, he might have certain expectations about how a young witch should behave. If Hermione violated these in some way, like disrespecting an old magical custom, Sirius would scold her.
She might initially be defensive. Hermione is proud of her intelligence and actions, so she could start by trying to justify what she did. For example, she could say that she was only trying to help or that she had good intentions.
Often, it begins with Hermione trying to help Sirius deal with his post - Azkaban trauma. She uses her knowledge and compassion to reach out to him. As he heals, he starts to see her in a new light. Their relationship then grows as they start to understand each other better. The lemon parts usually are used to explore their more passionate and private moments, which are often written in a very tender and intense way, adding a new dimension to their relationship.
One popular story might be 'Forbidden Bonds' which explores a unique relationship between them. It delves deep into their characters and how they might interact in a different way than in the original series. The author creates a lot of tension and drama, making it an engaging read.
It could be that she's homesick. Even though she loves being at Hogwarts, sometimes she might miss her muggle family a great deal. Also, she might be crying because she's being bullied. There could be some students who are jealous of her intelligence and are giving her a hard time. In the fanfiction, Sirius finding her crying might lead to him becoming a sort of mentor or protector for her, helping her deal with whatever is making her sad.
One reason could be that Hermione is under a lot of stress in the fanfic, and she takes it out on Sirius. Also, she might have some misinformation about Sirius that makes her think less of him. For example, if there are rumors going around about Sirius' past actions, Hermione might believe them without really getting to know him better. Another possibility is that Hermione is trying to assert her authority in a situation where she feels it's being challenged, and Sirius just happens to be in her way.
Their own personal histories could also be a factor. Remus has always been an outsider in some ways because of his werewolf nature, and he might see Hermione as someone who accepts him completely. Sirius, on the other hand, has had a troubled past and Hermione's loyalty and goodness might be very appealing to him. So, they could fight over who 'deserves' her more based on their own self - perceived worth. And also, they might be fighting not just for Hermione but for a sense of normalcy and a future that she represents in their lives.
The 'Sirius and Hermione make love' concept in fanfiction is controversial because it violates the integrity of the original 'Harry Potter' story. The series has a well - defined set of relationships and character dynamics. Sirius and Hermione have no such relationship in the source material. Moreover, the age and maturity difference between them is a big factor. Hermione is in the process of growing up and Sirius is already an adult with a complex past. A sexual relationship between them in fanfiction not only goes against the moral and ethical framework of the real world but also disrupts the fictional world that Rowling created.
One reason could be to create drama. Fanfiction writers often like to add a lot of angst and turmoil to the story. By having Ron cheat on Hermione, it can create a huge rift not only between them but also among their friends. Another reason might be to explore different character traits. Maybe showing a darker side of Ron that wasn't explored much in the original books.
In a Hermione and Sirius family fanfiction, Hermione might end up marrying into the Black family. Since Sirius was an important part of the Black family before his estrangement, Hermione could find herself involved in the family's complex politics and traditions. She could use her knowledge of magic and her Muggle - born perspective to challenge some of the old - fashioned ideas within the family. Sirius, on the other hand, could be like a mentor to Hermione, teaching her about the more secretive and dangerous aspects of the wizarding world that she might not have known otherwise.
There could be a fascination with power dynamics. Sirius is an older and more experienced wizard in the Harry Potter world compared to Hermione. Fanfic writers may be drawn to the idea of how his knowledge and power could interact with Hermione's intelligence and determination, thus leading to the concept of him dominating in the fanfiction. Also, it might add an element of unexpectedness or novelty to the relationship between two such different characters.