Instances in which one preindustrial civilization was a major factor in the collapse of another?Why was this so rare?
To be honest, I don't think it is really all that rare. There are numerous instances of preindustrial civilizations playing a factor in the collapse of others. The first one that jumps to mind is the Huns and their role in the fall of the Western Roman Empire in 476AD.
As the Huns continued their conquests throughout Asia and swept west into Europe they collided with barbarian tribes such as the Vandals and Visogoths. As a result, those tribes were forced west into conflict with the Roman Empire.
While there is little doubt that there were numerous causes for the fall of the Roman Empire, the final straw was the sacking of Rome which was a direct result of the Hun incursion into Europe.