Hypothesis 2 - Nutritional

9 month old large-breed dog is being fed free choice
Nutritional excesses may lead to accelerated growth rate which can lead to aberrations in ossification (low density, low resistance bones)
This can lead to developing cartilage and growth plate problems with premature closure of growth plates.
Diets excessively high in energy can lead to DOD.

Often puppies are switched from growth to adult maintenance diets under the pretense that it will avoid Calcium excess and skeletal abnormalities.
However, maintenance diets are lower in energy and the dog must consume more food on a DM basis to meet its energy needs. If the growth and maintenance diets are similar in Calcium levels (DMB), then the puppy may consume too much Calcium.
Calcium supplementation via table scraps / Iams diet + treats may cause transient hypercalcemia which leads to hypophosphatemia. 
Calcitonin is released in response to high Ca++ levels.
This leads to decreased bone resorption, which leads to thickened cortical bone, and interferes with normal bone remodeling.