Can be 1 0f 2 things really, either sudden heart failure or congestive heart failure. 1st one basically heart attack and drop dead, no warnings or 2nd , the heart starts to get weak and then it gets bigger, then you get the ‘cough’ fluid on the lungs and they have trouble breathing, lose weight and eventually die :'(
There is treatment nowadays but basically you are jusr prolonging life for about 3-5 months if your lucky. Max died from this when he was 5 :'(, Aston was 7 :'( and Jay was 12 :'( so its not really an old dog thing either. I read once that in the US 10,000 Dobes die from DCM yearly :o.