I'm not a vet and don't know about what effects the combination of medications (anesthesia, gas, valium etc) given your dog may have had.
Without a necropsy, it's impossible to know what killed your dog. Death from anesthesia does happen, but I've never heard of it being "common".
Sometimes animals can have hidden defects, like heart trouble, and sometimes too much anesthetic is given, but usually the animal dies on the table.
If you really want to know what is responsible for this, a necropsy is the only way.
I'm so VERY sorry for your shocking loss.