[Rich in Tucson]

I think all of the above, but the fatigue factor is the ultimate determinant. The blades of the few broken ones I have seen were full of dust/dirt because of the opening at the hub and that traps moisture. When you look at some intact ones, you can sometimes see them bulging in places out on the edges due to this build-up and the expansion caused by the growth of rust. The extra weight in the blades accelerates the fatigue.