I'm working off my own memory of the discussions and postings when it happened, so really I'd just be googling it myself to give you any further readings.

She was a motorcycle racer and this was her first job in stunts. So while she was technically experienced, the bike they used for the movie was way stronger than she was used to and they just forgot to set up barriers to keep her from flying off set, which is how she would end up crashing. Because they outright admitted in court to having basically given zero fucks to safety that day.

Which they knew she wouldn't be ready, because they imported a random black racer from Brooklyn all the way to Vancouver "for some reason" (we know why).

All the rest is reading between the lines, which I suppose means its not "confirmed" but we are all smart enough to see why many of the decisions were made.