Just as a random thing, I was actually able to identify the point of failure this time. There was a piece with an ugly lump in it that apparently I hadn't compensated for sufficiently. Eventually gravity won, the back stack failed at that point (at about knee level,) pitched forward and wiped out the front stack on the way down.
It's all off the lawn. We won't need to order wood next year. It was just so damn warm this year, and I ordered for a winter like the previous one. So we're set for a bit. Which is unfortunate, because now if I decide to paint the garage, I need to break down the stack.
Weather's supposed to be crappy tomorrow, so I can start raking on Wednesday.