A cervical spine injury, an injury to the high upper back or neck, can easily happen in elderly people as the result of a fall. The seven vertebrae (bones) in the neck are delicate and the angle at which your mother hit her head was likely how and why this happened.
Why did she die? Without autopsy findings, it would not be possible to tell the cause of death, but in general, there are some reasons why someone may die as a result of breaking their neck. One cause would be the inability to breathe. If the injury was at or above the fifth vertebrae, or if swelling occurs and presses on that area, it can affect the ability to breathe. The other cause would be a sudden and severe drop in blood pressure, causing the body to go into shock.
Another issue to consider is although your mother may have seemed perfectly healthy, there could have been some health issues that were not diagnosed, that she didn’t know about. It could be that this contributed to the problem.