Well, it could be that the girl is in the middle of a schoolyard fight in the fanfiction world. She might be trying to defend herself or someone else. Then, out of the blue, an opponent's quick movement causes her glasses to be knocked off. It's a crucial moment as it can symbolize her vulnerability. Without her glasses, she has to rely more on her other senses and instincts to continue in the fight or deal with the situation.
In the fanfiction, let's say it's a fight in a magical realm. The girl, who is not much of a fighter, is dragged into a conflict. As the fight intensifies, a spell misfires or a magical creature bumps into her, and her glasses are knocked off. This not only adds a sense of chaos to the scene but also shows her being pushed out of her comfort zone. She has to quickly adapt, whether it's by using a magic spell to retrieve her glasses or learning to fight without clear vision.