In researching the lines, look for congentital heart defects.
I agree with IBBoykin.....this is the most likely cause, IMHO, given the suddeness and age of the dog. Most likely some type of dysrhythmia if there is no sign of trauma, ect.
I had a friend who got her dogs(GWP) from a particular breeder; of the three she got from them, 1 had a dysrhythmia and died at a young age, one had a heart murmur that prevented the dog from trialing (which is why she bought him in the first place) and the third was just plain nuts!
While the autopsy might be pricey, it might uncover something unforseen....I used to work in a veterinary hospital back in the early 80s; one day a lady brought her Pomeranian in because the dog wasn't acting right: restless, panting, wouldn't "calm down". My boss (who I think is a brilliant clinician) examined the dog and orderd a chest X-ray...what we found was that the dog had been shot in the chest with a pellet gun! The entrance hole was so small, it could have been passed over as just a scratch, and there was no blood to speak of.
Can't say that an autopsy will give you something like that, but it might bring some closure for your family. Sorry for your loss..............