Wasn't the accident per se, but the type of injury (head), that caused the sad death of this young woman.
Basically any motion or injury that causes the head to sharply move fowards and or backwards can cause the brain to bounce about and or hit the skull. When this happens there can be damage to parts of the brain including the blood supply. Because the damage may only occur at one part of the brain at first, the other parts may seem to work perfectly, which is why Miss. Richardson was reported to be acting "fine" right after the injury and while still in Canada.
These sort of accidents are the rationale behind wearing protective head gear, including helments for many sports and other things that can cause a fall and or head injury. Bike riding, roller blading, horseback riding, ice skating and so forth fall under this category.
L.