First off, she would be killed instantly. There wouldn't be any question of what might kill her "ultimately" because ultimately would be the moment she hit the ground. Yes, there have been people who survived falls from much greater heights, with the record holder being Vesna Vulović, a Serbian flight attendant who fell 10,160 m (33,330 ft), but those are vanishingly rare incidents and most involved a great deal of luck (hitting things that slowed the fall such as trees and power lines, landing in snow drifts, etc). In general, an 80-90 foot fall onto a hard surface is certain instant death 99.99% of the time.
Since she lands on her back, what will kill her most quickly is the massive head injury. Autopsy will find a crushed skull, massive brain injury, massive spinal cord injury, and possibly a detached brain stem, which would be instantly lethal.
The second thing that will kill her is deceleration. When she hits the ground, her body stops its downward motion instantly, but Newton's First Law of Motion says her internal organs that are free to move will keep going. They'll keep traveling in the same downward direction at the same speed. The result of that will likely be the transection of her aorta, which will cause immediate exsanguination. This is because parts of the aorta are free to move and other parts are anchored in place:
The aorta has some fixed regions, particularly the relatively fixed
aortic arch (to the thoracic inlet by the brachiocephalic vessels).
The remaining portions of the aorta are relatively free. Ascending and
descending aorta are relatively mobile. These can result in an unequal
distribution of shear forces on the relatively fixed arch and mobile
ascending and descending aorta and stress at the site of attachment of
the aorta, particularly the aortic root and the aortic isthmus.
As if the victim isn't dead enough by these two things, there will be a wide ranging assortment of other injuries, many of them lethal in and of themselves. The coroner will most likely find:
- Virtually every bone in her body will be fractured, including her skull, spine, ribs, pelvis, long bones of the arms and legs, scapulas, and clavicles.
- Her liver will be transected by the falciform ligament and the same deceleration forces that transected her aorta. This will also result in massive bleeding.
- Deceleration injury will be widespread in almost all the other organ systems such as the lungs, spleen, intestines, and other blood vessels large and small.
In general, even if a full team of trauma surgeons with a fully equipped OR was scrubbed in and ready right at the scene, she would still be dead on impact and nothing they could do would change that.