I've seen this type of material degradation numerous times, though not necessarily in suspension bushings. It seems that plasticised rubbers (or rubberised plastics, not sure how that works) have a definite shelf life, and once that time is reached, they just fall apart. There is a brand of safety work-boot that is popular in this area with a sole that is as tough and resilient as a car tire, until it reaches ten years old, at which point, worn or not, it will just fall to bits while sitting on the floor. I've seen many other instances of this phenomenon, including the tires of the model cars I build, which when decades old will either harden and crumble, or leach out an oily substance (the plasticiser), just sitting on the shelf. The problem, at least in terms of very short shelf life, seems prevalent to modern-day products, suggesting the use of a new breed of inexpensive, high-performing materials that don't have longevity. Pretty much like everything these days, it seems.
Relative to our cars, I have had a pair of Stromberg diaphragms fall apart when fitted to the carbs. The carbs had been rebuilt and then sat on the shelf for twelve years. In diagnosing why they wouldn't work when fitted, I found both diaphragms decomposed and falling apart. Clearly an inferior copy of the product, and I am pretty sure they perished in this same manner.
It's something to keep in mind with all plasticky rubbery bits, especially in this day of reproduction parts where inexpensive and untested materials are employed that work in the short term, but don't have the stamina to go the distance.
