One possible reason could be that she might think Harry is putting himself in unnecessary danger, like when he goes off on his own to face Voldemort without a proper plan. Molly is very protective, and she may feel frustrated with Harry's recklessness.
Well, in some fanfictions, if Harry were to say or do something that she perceived as disrespectful towards her family or the Weasley way of life, Molly might lash out. For example, if he were to make a comment about Ginny that she didn't like. Also, if Harry were to bring dark magic related things too close to the Burrow or her family, given her fear of the dark arts, she could react with a hit. It's all about her strong maternal instincts and the need to protect her family and the values they hold dear.
Perhaps Harry makes a decision that she believes will harm Ron or Ginny. Molly loves her children deeply and if she thinks Harry's actions could lead to their pain or misfortune, she might hit him. In the wizarding world, actions are sometimes met with physical reactions, especially when emotions are running high. Molly is a passionate woman, and if Harry does something that goes against what she thinks is right for her family, her anger could lead to her hitting him.
One possible reason could be a misunderstanding. Maybe Molly does something that Harry misinterprets as a threat to his friends or his mission. For example, if Molly tries to keep Harry from a dangerous but necessary task, Harry might see it as her overstepping and feel the need to 'punish' in the context of the fanfiction, which could just be a strong rebuke or some form of isolation within the story's plot.
One reason could be that the fanfiction changes her personality traits. In the original, she's kind - hearted but in fanfic, she might be made mean. Harry, who knows the real Molly, won't like this new version.
One possible reason could be that Molly might blame Harry for putting her family in danger. Since he is often at the center of Voldemort - related troubles, her family gets involved in protecting him. For example, during the Battle of Hogwarts, her family was right in the middle of the fight mainly because of Harry. Another reason could be that she blames Harry for Ron's decisions at times. Ron follows Harry on his adventures and sometimes it puts Ron in harm's way, which Molly as a mother may not like.
In some more extreme fanfiction scenarios, she might use a magical object like a broomstick to swat at Harry. However, this would be a very out - of - character action for Molly in most cases, but in the realm of fanfiction where creativity runs wild, it could be a possibility. She would probably only do this if she was extremely angry and felt that Harry had really crossed a line, like endangering one of her children's lives in a really stupid way.
One great Molly Weasley fanfiction is 'Molly's Moments'. It shows her behind - the - scenes efforts in taking care of the Order of the Phoenix members at the Burrow. Another is 'Molly's Motherly Love' which delves deep into her relationship with each of her children, especially during the tough times of the wizarding war. There's also 'Molly's Magic' that focuses on her underrated magical abilities.
One possible storyline could be that in an alternate universe, Harry Potter and Molly Weasley develop a relationship over time. Maybe Harry is in need of a motherly figure after all he has been through, and Molly's nurturing nature draws them closer. They might face opposition from the wizarding community as it goes against the norms.
In fanfiction, Molly Weasley is often depicted as a strong - willed and loving mother figure. She is shown as someone who is extremely protective of her family, just like in the original books. For example, she might be portrayed as being very strict when it comes to the safety of her children, especially during dangerous times in the wizarding world.
The relationship could start with mutual respect. Harry has always respected Molly as a mother figure in the Weasley family. This could gradually turn into something more romantic as they share more personal experiences. For example, Harry might confide in Molly about his post - war traumas, and Molly's understanding and care could lead to deeper feelings.
It could be that Harry made a decision that went against Molly's values. Suppose Harry started using dark magic or got involved with some unsavory characters, Molly, being a morally upright witch, would be furious with him. She might feel that he is setting a bad example for her kids or bringing danger to their world.
One scenario could be that Harry Potter accidentally breaks a family heirloom of the Weasley family while at the Burrow. Molly, who treasures her family's possessions, would be very angry with him. Another possibility is that Harry gets into a fight with one of her sons, like Ron, and Molly takes her son's side, getting mad at Harry for starting the argument.