Originally Posted by Drew Eckhardt

Counter-intuitively, bicycles stand on the lower spokes which see a decrease in tension with minimal changes elsewhere in the wheel.

Barring damage (like shifting the chain into them) spokes fail due to fatigue, with the number of cycles dependent on average stress (where the elbows in non-stress relieved spokes have areas with high average stress because they were never taken past their elastic limit) and magnitude of the variation (which is obviously higher for bigger riders and/or with luggage).