Fanfiction allows for a lot of creativity. Perhaps the story is set in an alternate universe where Snape and Hermione have a different dynamic. Snape could have redeemed himself much earlier in this universe, and Hermione has come to understand his true nature. So when he holds her, she feels safe because she knows he is on her side, that he has changed from the cold and strict professor she knew in the original story.
One possible reason could be that Snape has shown unexpected kindness to Hermione in the fanfiction. Maybe he has protected her from some danger secretly, which makes her feel she can rely on him and thus feel safe.
One scenario could be that Snape saves Hermione from a Dark Arts spell gone wrong. As he catches her in his arms after deflecting the spell, she feels safe because he has just rescued her life.
Often, it starts with a shared experience. For example, they might face a dangerous magical creature together. After that, Snape might show concern for Hermione's well - being, which begins to build her sense of safety around him.
In the original series, Harry and Snape have a very tense and often hostile relationship. But in this fanfiction, their relationship is likely to be much more positive. Harry feeling safe with Snape means there is a sense of trust that was absent in the original.
One reason could be Snape's knowledge. Hermione, being a knowledge - seeker herself, might feel safe knowing that Snape has a vast understanding of magic, especially the darker aspects. So, she believes he can keep her safe from any threats related to that knowledge.
One reason could be the re - imagining of Snape's character. In fanfiction, he might be kinder and more understanding towards Hermione than in the original story. So, she feels safe in his presence.
One reason could be that in this fanfiction, Snape has some hidden kindness towards Harry that he doesn't show in the original story. Maybe he secretly protects Harry from other threats, which makes Harry feel safe around him.
In the original series, Snape is often quite harsh towards Hermione. However, in this fanfiction, the relationship is completely different. Here, Hermione feels safe with Snape, which is a huge deviation as in the original, she would be more likely to be on guard around him.
Well, in a Hermione marries Snape fanfiction, their relationship could start from an unexpected understanding. Maybe Hermione starts to see beyond Snape's cold exterior as she delves deeper into potions, his area of expertise. She realizes there's more to him than his strict teaching style. And Snape, on the other hand, might be intrigued by Hermione's thirst for knowledge and her unwavering determination. This could gradually lead to mutual respect and, over time, develop into love.
In the fanfiction, Snape may protect Hermione by shielding her from the Dark Lord's influence. Given his double - agent status, he would be in a position to know if Voldemort has any plans involving Hermione. He could then take steps to keep her out of Voldemort's sight, perhaps by misdirecting Death Eaters or using his own magic to create a protective barrier around her when she's in danger.