The most common cause of a dog dying suddenly is heart disease or cardiovascular disease.
Other things that can cause a dog to die suddenly are toxicity, gastrointestinal disease, trauma and non-traumatic hemorrhages.
Dogs already have a pretty short lifespan and it can be a sudden thing that a dog dies even before they are expected to die.
It can also be hard to tell or know whether or not the dog has heart disease, blood clots etc or is about to have a heart attack.
That's why if you can afford to it's important to take the dog to the vet for regular checkups so that if your dog does have heart disease it can be treated.
Not everyone can afford to take their dog to the vet though even though they love them dearly and that is okay.
Heart problems, Blood clots, abnormal heart rhythms, cardiomyopath etc are all causes of sudden death in dogs.