Pretty low mileage "given it's age" might mean something in the neighborhood of 30,000 miles on a bike that age. Yeah, that's about 20mi/wk, which is pretty darn low for a road bike.

And 30,000ish miles is enough to develop some fatigue in any thing as load bearing as the rear axle.

So, like a coroner, I'd put the cause of death down as "natural causes".