I am so very sorry for your loss. I can't imagine what you must be going through. That isn't uncommon to happen, I have seen in a few times since I started working in vet medicine over the past several years. It is difficult to say what exactly causes it, but many times the one to die later usually suffers from Takotsubo cardiomyopathy or stress cardiomyopathy, also known as broken heart syndrome, where a portion of the heart enlarges. It is usually a temporary condition and one heals, but it can also go the other way. This happens with humans as well and many times they can not always find a clear cut answer to why the person died. Again, I am very sorry for your loss.