Perhaps Itachi is under some sort of mind - control or false pretense himself. He may not be acting entirely of his own free will and tying the ropes around Sakura's wrists is a result of this external influence. It could also be that he is trying to teach Sakura a lesson about trust and obedience, in a rather extreme way.
Itachi might be doing this as a means of restraint to prevent Sakura from interfering in a dangerous situation. He may know that she has a tendency to rush into things without thinking, and this is his way of ensuring she stays put. Another idea is that it could be related to a cultural or ritualistic practice within the context of the fanfiction's world.
Sakura could also be confused. She may try to reason with Itachi, asking him why he is doing this. If she trusts him to some extent, she might be more compliant while still expressing her discomfort. She may start to analyze the situation, trying to figure out if there is a hidden meaning or purpose behind his actions.
It could be a story where Itachi has some sort of ulterior motive for restraining Sakura. Maybe he's trying to protect her in a very unorthodox way. For example, he might be aware of a danger that only he can sense, and by restraining her wrists, he's ensuring she doesn't run into harm's way.
It could be Orochimaru. He's always had an interest in powerful shinobi, and Sakura has shown great potential. Itachi might be roping Sakura's wrists to prevent Orochimaru from getting to her and using her for his evil experiments.
Fanfiction is fictional stories written by fans. In this case, it might be about a scenario where Itachi pins Sakura's wrists. It could be part of an action - filled, or perhaps a more complex and dramatic story within the fan - created world of the relevant anime or manga.
One motive could be that Itachi was trying to protect a greater secret or a more important person. He might have calculated that sacrificing Sakura's safety in that moment was worth it to keep something else safe. Or perhaps he had some sort of vendetta against Sakura or those associated with her. He could be using this as a way to get back at them. However, this is a very dark and immoral motive, but in the context of a fanfiction where such a scenario is presented, it's a possible explanation.
I'm not sure if it's a particularly popular type. There are so many different kinds of Naruto fanfiction out there, focusing on various relationships and scenarios. While the idea of Itachi and Sakura interacting in this way might be interesting to some, it may not be as widespread as other more common pairings or plotlines.
One possible motive could be to use Naruto as a bargaining chip. Itachi was a complex character with his own goals related to the safety of the Hidden Leaf Village in some ways. He might have thought that by kidnapping Naruto, he could influence the village's decisions. For example, he could try to stop certain actions the village was planning that he believed would lead to more harm. Another motive could be related to the Nine - Tails within Naruto. Itachi might have wanted to study or control the power of the Nine - Tails for his own purposes, perhaps thinking it could be a way to protect Sasuke in a different manner.
One reason could be that he sees something special in Sakura. Maybe he admires her strength or her kind heart, and he doesn't want to lose her to someone else who might not appreciate her as much as he does.
One possible reason could be that their different loyalties in the ninja world caused a rift. Itachi was deeply involved in the secrets of his clan and the village, while Sakura was mainly focused on her friends and the well - being of Konoha in a more straightforward way. Their priorities might not have aligned in the long run, leading to a breakdown in their relationship.
One possible motive could be to protect Sasuke. Itachi might have seen a great danger approaching Sasuke that he could only avoid by taking him away forcefully. Maybe there was a powerful enemy targeting Sasuke, and Itachi thought that by kidnapping him, he could keep him safe in a hidden location and train him to become stronger.