Why do stock 10-bolts fail — housing flex or shock loading?

Speedway points to shock loading: drag radials and slicks put a real shock into the axle and driveline, which is why its ratings drop sharply with stickier tires. The smaller 10-bolts also have 28-spline axles with no 30-spline upgrade.

GM 10-bolt	Stock capacity (Speedway)	Notes
8.5 / 8.6	The only high-performance 10-bolt; can hold about 1,000 hp built properly	Large square lugs at 5 and 7 o'clock; flat webs on the front; 28- or 30-spline
8.2 (1964–72)	About 400 hp on street tires, 350 on drag radials, 300 on slicks	Built for racing: about 500 hp on street tires, 400–450 on slicks
7.5 / 7.625	About 350 hp on street tires	Maxes out around 500 hp with maximum effort and expense; 28-spline only, no 30-spline upgrade

We don't have a source that measures housing flex, so we can't weigh the two directly.