One possible reason could be for a power play. Draco, being from a Slytherin family, might want to show his dominance over Harry, who is seen as a hero in the wizarding world. By tying him up, he could be trying to intimidate Harry or gain some sort of advantage in their ongoing rivalry.
Harry could initially be angry and struggle. He's a brave character, and being tied up against his will would surely make him fight back. He might yell at Draco, demanding to be released and threatening consequences.
One possible plot reason could be a misunderstanding. Maybe Draco thinks Hermione has some information that she's not sharing and he ties her up to force her to talk. In some fanfictions, it could be part of a game or a dare within the magical world they inhabit. Another reason might be that Draco is being influenced by some dark magic or a curse that makes him act out of character and tie Hermione up.
One possibility could be a story where it's part of a magical prank gone wrong. Maybe Draco was trying to play a joke on Hermione during a school event at Hogwarts. He ties her up using a charm that he thought would be harmless, but then they both end up in a situation where they have to work together to undo the charm. This type of fanfic would focus more on their relationship development through this comical yet challenging situation rather than anything inappropriate.
Hermione might initially be furious. She is a strong - willed character, so being tied up against her will would surely make her angry. She would probably start using her wits right away, trying to figure out a way to free herself, whether it's through magic or by tricking Draco.
Harry might initially be shocked and angry. He would probably struggle against the restraints and demand to know why Snape is doing this. He could yell at Snape, asking for an explanation and protesting his innocence if he believes he has done nothing wrong.
One interesting plot point could be the reactions of their friends. Harry's friends might be really confused seeing him with Draco's tie, and Draco's cronies might be equally shocked. It could lead to some humorous exchanges as they try to figure out what's going on.
In this fanfiction, the tie - switch can be a tool to explore their insecurities. Harry might feel a bit self - conscious wearing Draco's tie, worried about what it says about him. Draco, on the other hand, might be afraid of losing his 'pure - blood' image if his tie is on Harry. This can lead to some great character - development moments.
It could be that Harry has a new spell or technique that Draco is not aware of. Maybe he got some special training from a powerful wizard. Harry could use his agility and speed to dodge Draco's attacks and then counter with a series of powerful spells. His ability to think on his feet and adapt to the situation during the duel would also play a big role in showing Draco up.
Well, Fanfiction.net is a popular place. There are so many different fanfictions there. You might also want to check out some Harry Potter - focused fanfiction communities on social media platforms. Sometimes, members of these communities share links to interesting fanfictions, including ones that might involve the scenario you mentioned. Another option could be to ask other Harry Potter fans you know if they've come across any such fanfictions.