In this case the nut isn't the initial failure point. It seems, based on what
I checked based on reports from a few different people that the saddles have almost a W shape, where there's a raised ridge at the bottom of the radius.
There's some pictures earlier in the thread showing the issue, and some of the aftermath of the shafts coming loose.
With that much pressure on such a small contact surface, it seems that the peak is eventually flattened out by the shaft. But with the side effect of losing torque on the nuts.