Was out on a group ride today, and at one point we went down a short (maybe half a mile) but steep (varying between 5-10 % grade) hill. At the bottom of the hill, a sharp bang rings out, and the guy next to me yells "Flat!".
I could see, even before he had come to a stop, that the rear tyre was completely off the rim. I helped him put new tube in, and checked the old. It was split open along its length, for about one fifth of the circumference.
My analysis: He used his rear brake alone throughout the descent. This heated the rim, causing the pressure to increase. This popped the tyre off of the rim, which let the tube out, free to expand unchecked, and then split.