Maybe it was a jinx. In the wizarding world, there are many spells that could make a broom go haywire. Some jealous rival in a Quidditch match could have cast a spell on his broom to make him fall.
It could be due to internal turmoil. Regulus was in a complex situation, being involved with the Death Eaters. His inner conflict might have distracted him during the game, causing him to lose control of his broom and fall. For example, if he had just received some disturbing news about Voldemort's plans, it could have affected his concentration.
Perhaps it was a malfunction of the broom itself. Brooms can be temperamental in the wizarding world. Maybe it was an old broom that he had borrowed or a new model with some design flaws. The broom could have suddenly lost its magical balance and sent him tumbling down.
There's a fanfic titled 'When Regulus Black Took a Tumble'. In this, the fall off the broom is not just a simple accident. It's part of a plot where someone has jinxed his broom. The story then follows Regulus as he tries to figure out who is behind it and why they would target him.
He could get seriously injured. In some fanfictions, he might break a bone or two. It would also likely lead to him being out of the Quidditch match if it happened during one.
In some fanfictions, his fall off the broom could be a turning point in the story. It might lead to him discovering a new magical ability. For instance, he could unconsciously use a form of magic to slow his fall or even levitate himself slightly. This newfound ability could then be explored further in the story, perhaps making him a more powerful wizard in unexpected ways.
One possible reason could be to protect himself from the pain and loss he has endured. After all he's been through, like the death of his parents, Sirius, and so many others, turning off his emotions might seem like a way to avoid further hurt.
It could contain accusations. For instance, Regulus might accuse Voldemort of betraying the pure - blood ideals he once claimed to uphold. He could point out how Voldemort's actions were actually harming the wizarding world rather than purifying it.
One reason could be a misunderstanding. Maybe Kate did something that Tony misinterpreted, leading to him telling her off. For example, if she made a decision without consulting him in a shared project within the story's universe.
It could also be a part of his training or self - discovery. Naruto might have realized that he was relying too much on his chakra and wanted to find a new way to be strong. By sealing off his chakra, he could focus on other aspects like his physical strength, strategic thinking, or even his ninja skills without the use of chakra.
Neville Longbottom might also be a character. Neville has his own connection to the fight against the dark side. He could share his experiences with her and they could bond over their shared desire to break free from the dark legacies of their families. Also, Luna Lovegood could be in the story. Luna's unique perspective on life and magic would be a great contrast to the daughter's more complex family situation, and they could have some very interesting conversations.
Maybe someone bumped into the shelf and knocked the book down. Simple as that!
One possible reason could be to save someone else. Maybe a friend or a loved one is in danger at the base of the tower and Harry believes jumping is the quickest way to reach them. For example, if there's a dark spell about to be cast on Hermione and the only way to stop it in time is for Harry to reach her instantly.