Perhaps it's part of a power play. Draco comes from a pure - blood family and Hermione is a Muggle - born. In some fanfictions, this power dynamic is explored more deeply. By cutting her hair, he is trying to assert his superiority over her. It could also be an impulsive act in the heat of an argument. They might be arguing about something like blood purity or house points, and he lashes out in a very physical and mean - spirited way.
Hermione could also be very hurt. Her hair is part of her identity, and having Draco, who she already has a strained relationship with, do something so cruel would cut deep. She might go to her friends, Harry and Ron, for support. They would likely be just as angry as she is and plot some sort of revenge against Draco and his Slytherin friends.
One possible reason could be their different backgrounds and values. Hermione is a Muggle - born witch who values knowledge, justice, and equality. Draco, on the other hand, comes from a pure - blood family with a long - standing tradition of pure - blood supremacy. These fundamental differences might lead to constant clashes in their marriage, causing them to grow apart and eventually divorce.
In the fanfiction, it could be related to family values. Draco comes from a family with certain beliefs about blood purity. Hermione being a Muggle - born might be seen as beneath him by his family's standards. So, Draco might abuse Hermione as a way to show his superiority based on those wrong family - instilled ideas.
It might be due to a dare or a bet in the fanfiction. Her friends could have challenged her to cut her hair, and being the bold character she is, Sakura accepted. This could lead to a series of events where she discovers a new side of herself, like feeling more confident or independent without her long hair.
One possible reason could be that Draco has had a change of heart. Maybe he realizes that Hermione is a person worthy of protection regardless of her House or her friendship with Ron. He might have seen some injustice about to befall her from Ron and his Gryffindor bravado getting out of hand.
There could be several reasons. Draco might be facing some sort of blackmail. Someone could be forcing him to be with another person for some hidden motives. Also, his own pride could play a part. He may not want to admit that he's falling for Hermione, a Muggle - born in the eyes of some in the wizarding world. So, to prove to himself that he doesn't care about her as much as he thought, he cheats. Additionally, he could be influenced by the peer pressure within the Slytherin house, where some might not approve of his relationship with Hermione and push him towards someone else.
In some fanfictions, their fight might be due to a competition at Hogwarts. Maybe they are both vying for the top spot in a particular class or for a special award. Hermione's studious and competitive nature combined with Draco's pride and desire to outdo others could easily result in a fight.
One possible reason could be to get her attention in a very forceful way. Snape is often a complex character in fanfiction, and he might be trying to make Hermione listen to him urgently, perhaps about a dangerous situation in Hogwarts that she's not aware of.
Some writers might do it to explore the darker sides of the characters. They want to see how Hermione, who is usually so strong and intelligent, would react in such a terrible situation. And for Draco, it could be a way to show his capacity for evil in a more extreme way than in the original books.
Maybe it's a magical protection. In the wizarding world, there could be some ancient magic associated with hair length. Long hair might act as a sort of shield or a connection to some deeper magic that Harry unknowingly benefits from.