It's pretty sad, but I think the majority of all deaths are the owner's own fault, and in many cases could have been prevented...
Think of causes such as wrong acclimatisation of fish (if at all), intentionally overstocking without knowing what it takes to do so, point- and needless experiments at the expense of the fish, turning the tank into a chemical cocktail, feeding the wrong food or too much, sloppy maintenance, not knowing enough to care for their fish properly, not knowing about essential things such as tank cycling and water chemistry, etc. etc.
If people can keep their piranha('s) alive for a decade or more without too much trouble (and some do, as some of our members illustrate), we all can - still it's a rare occurence piranha's get that old...
The amount of fish that actually die of "natural" causes - such as diseases, parasites or old age - or just dumb luck is pretty small.