Since I assume that a post-mortem has not been performed, than, your dog
may have had a congenital abnormality (malformation) of
, or vessels of
that was not compatible with long life. This would have been difficult or impossible to foresee, especially since it sounds as if the problem had no warning signs, i.e.: prior episodes of sudden shortness of breath, or acute collapse/fainting episodes, or heart murmur.