Well, it could be that Naruto made a really stupid mistake that hurt Hinata's feelings deeply. Maybe he accidentally said something really mean about her family or her skills in front of everyone. So in a fit of anger, Hinata slaps him to show her extreme disappointment.
One possible reason could be a huge misunderstanding. Maybe he thought Hinata did something really bad to his friends, like unknowingly leading the enemy to them. Without knowing the truth, his hot - headed nature made him lash out.
One possible plot could be that Naruto and Hinata find out about the pregnancy and they are both overjoyed. They start preparing for the baby's arrival, with Naruto being super protective of Hinata. He might go on missions less often to be around her. Hinata, on the other hand, could be seen knitting baby clothes and daydreaming about their future family life.
It could also be a misunderstanding. Hinata might be under some sort of mind - control or false impression. For example, an illusion is cast on her that makes her believe Naruto has become evil. As a result, she takes actions against him that he perceives as betrayal. In this case, once the truth is discovered, they would likely reconcile.
Well, perhaps in the fanfic, Naruto is away on a mission when Hinata discovers she's pregnant. She then has to deal with the initial shock and excitement all by herself. But she's strong, and she starts taking care of herself and the unborn baby. When Naruto returns, he's overjoyed and vows to protect his family even more fiercely. And they could have a little ceremony in the village to celebrate the upcoming addition to their family.
At first, Naruto would be shocked. He might be angry at Hinata for slapping him. But then he would start to think about why she did it. If he realizes his mistake, he could become more cautious around her. Hinata, on the other hand, would probably feel a bit guilty for slapping him, but also relieved that she got her feelings out.
One possible plot could be that Boruto does something really reckless, like using a dangerous jutsu without proper training. Naruto, as his father and the Hokage, slaps him to snap him out of his overconfidence and make him realize the seriousness of his actions.
One possible plot could be that Ino says something really offensive about Naruto's dreams or his past. For example, she might mock his early days as a weak ninja in the academy. Naruto, who has grown and become more confident, slaps her in a moment of anger to defend his pride. Then, he realizes his overreaction and feels guilty, leading to a story arc about making amends and understanding each other better.
Maybe in this fanfiction, Naruto is in an alternate reality where he doesn't know Kushina is his mother. He might be misled by false information about Kushina being a threat. And when he confronts her, his hot - headed nature gets the better of him and he slaps her. But then, as the story progresses, he discovers the truth and regrets his actions.
Hinata might be shocked at first. She has always adored Naruto, and this sudden act of aggression from him would leave her stunned and unable to process it immediately.
Well, one possibility could be a plot where Hinata is under some sort of mind - control or dark magic. This makes her act out of character and hurt Naruto. Maybe she's being manipulated by an enemy who wants to break Naruto's spirit by using the person he loves. Another idea is that there's a huge misunderstanding between them. For example, Hinata might think Naruto has done something unforgivable to her friends or family, so she lashes out at him in anger, but it's all based on false information.