Another reason might be related to a new relationship or friendship Hermione has formed. If she has befriended someone who doesn't like Harry, she might be influenced by that person's views and start treating Harry badly. But this would also show a lack of her own strong character that we know from the main series.
One possible reason could be miscommunication. Maybe Hermione is so caught up in her own thoughts and studies that she accidentally snaps at Harry without realizing how harsh she is. For example, she might be under a lot of pressure to do well in her classes and Harry's requests for help or just his presence might seem like an annoyance at that moment.
One reason could be jealousy. Maybe Hermione is getting more attention in school for her academic achievements, and Harry, feeling overshadowed, acts mean. For example, in some fanfictions, he might make snide remarks about her always having her nose in a book.
Perhaps Draco has developed some sort of complex feelings towards Harry. He could be secretly attracted to Harry and treating him like a baby is a way to get closer to him without admitting his true feelings. Or it could be that Draco has changed as a person and now views Harry as someone who needs his support and guidance, just like one would for a baby.
One possible reason could be that Voldemort sees Harry as a symbol of innocence that he wants to corrupt. Just like how he enjoys toying with his enemies, treating Harry like a baby might be a way to belittle him and make him more vulnerable in his eyes.
The relationship between all three could become more complex. Hermione's bad treatment of Harry might make Ron more protective of Harry, causing a rift between Hermione and Ron as well. In the original series, they are a tight - knit trio, but in this fanfiction, the balance is thrown off. For example, they might not be as effective in fighting against the dark forces or solving mysteries together because of the tension.
In some fanfictions, there might be a new magical element or prophecy that makes Harry question Hermione's motives. If the prophecy says something about Hermione being involved in a dark plot, even if it's false, Harry may start to have doubts about her.
One possible reason could be a misinterpretation of their relationships. Maybe the author wants to explore a different dynamic where Harry and Hermione bond over Ron's perceived flaws, like his occasional laziness or his insecurities that might come across as annoying to them in this fictional scenario.
Jealousy can also be a factor. Maybe Harry becomes jealous of Hermione's close friendship with another character, or vice versa. This could create tension and ultimately result in a break - up. In some fanfictions, Hermione's intelligence and success might overshadow Harry at times, causing him to feel insecure in the relationship.
Power could be a reason. If there's a chance for Ron and Hermione to gain a great deal of power by turning against Harry, some fanfictions might explore this. For instance, a new dark power might offer them something they've always desired, like the ability to reform the wizarding world in their own image. They might think Harry would oppose this, so they betray him to get what they want.
It's possible that Harry hates it because he has a certain image of Ron and Hermione in his mind. Fanfiction can sometimes create versions of them that are completely different from what he knows. For example, some fanfiction might focus on a love triangle that he doesn't think exists in their real relationship. This divergence from his perception could lead to his dislike.
One reason could be that Hermione is overly worried about Harry's safety. Since he's always in the middle of dangerous situations, she might think controlling his actions can keep him out of harm's way.