You must put a spotlight on it. A big spotlight.
This incident is a plot point, it is not a "hook".
You must show that your protagonist has no idea what happened, but she is being blamed. The bully's body is examined for cause of death, and it looks like she was a healthy and strong 14 year old that suddenly had a fatal stroke, a burst aneurysm. The coroner thinks that highly unusual in a 14 year old, but not unheard of. Aneurysms typically burst predominately in adults over 50, with some in their 40's, but very very seldom in young teens still growing.
The girl is deposed by police, her parents and an attorney are with her. She's scared, she cries, she swears she didn't do anything! She thought this bully was really going to hurt her, and all of a sudden, the bully just stops and drops.
Other students witnessing the fight are questioned, and a few say the same thing. One second the bully was angry, the next she collapsed to the ground and was motionless.
The principal of the school wants to suspend our protagonist, or expel her, for fighting. Her parents fight that, their daughter wasn't fighting she was being attacked, at worst trying to defend herself.
If you are in modern times, perhaps another student used their phone to record a video, and finally shows it to her parents, and they bring it to the police.
The matter is dropped.
The reader needs to know this is inexplicable, and believe the protagonist had nothing to do with it.
That is your plot point. It is a mystery, and the reader will keep reading to solve it.
If you want a foreshadowing moment, have the girl's parents, arguing with the school principal, show the principal the video. And one of the parents says, when the bully clearly drops for no explicable reason: "My daughter had a guardian angel that day. She did nothing wrong."
Maybe that's true, for many it is just a saying, a religious attribution for otherwise inexplicable luck and circumstances. Most readers will get that, even though it is close to the literal truth in your story, they won't be certain it is true.
Put a spotlight on it. It is a significant incident, a good mystery for readers to remember (a good "hook"), and you can foreshadow the future reveal in a way that doesn't give it away.
It won't be mistaken as bad writing, because clearly you are devoting pages to it and spotlighting the unexplained nature of the incident.