One possible motive could be that Narcissa has some sort of misunderstanding. Maybe she thinks Hermione is a threat to her family or has some crucial information related to the wizarding world that she wants to extract. Another reason might be related to the power struggle within the wizarding community. She might be using Hermione as a pawn in a larger game that involves different factions.
Perhaps Narcissa is acting out of desperation. There could be a situation where she believes Hermione holds the key to saving someone she loves, like her son Draco. By restraining Hermione to the bed, she can ensure that she has the upper hand in getting what she wants from Hermione. It could also be due to some long - held grudges or secrets in the wizarding world that involve Hermione, even if Hermione is not fully aware of them herself.
It's possible that Narcissa is being controlled or influenced by some dark magic or a more powerful wizard. She might not be entirely in her right mind when she restrains Hermione. In the complex world of fanfiction, there could be an external force compelling her to do this, and Hermione just happens to be in the wrong place at the wrong time. This could lead to a series of events where Hermione has to find a way to break free and uncover the truth behind Narcissa's actions.