Yes they are, it is quite common for manufacturers to push them for the use of rabbits and small animals because they control the odor. What happens is that they emit phenols that can cause liver and kidney damage not to mention the dust can lodge in their lungs as well and be an irritant to their eyes and even scratch them. Young baby bunnies can also be suffocated by them. Here is more info:
This is a simple search on the HRS web site, I typed in "pine shavings"
http://search.atomz.com/search/?sp-q=pine+shavings&sp-a=00062824-sp00000000
Here is another listing of articles:
http://homepage.mac.com/mattocks/morfz/rabcare.html#litterwarss
To attribute the death of an animal to anything a necropsy must be performed by the vet to determine the cause of death. Unfortunately there are many things to consider and many things that could have occured that caused its demise like a genetic defect, disease etc.