I think you built yours a little better than that particular one, your tire is mounted much lower and doesn't put as much leverage on the spindle. BTW, totally dig the stealth look of that setup, nice work!!
As for what happened to that one in particular, you can see from the close-up of the sheared spindle that it had been failing for awhile (rusty on the perimeter), and it finally gave away on that snow run as we were bombing over some whoops. In all fairness, that TJ had been running 37" Pit Bull Rockers on Hutchinson dual beadlocks for years, which is quite possibly the heaviest tire/wheel combination known to man, and this was one of the first trips after downsizing to some cheap 35's. I didn't build that one, but the guy that did was there that day and admitted he'd seen similar failures on a few others he had built the same way. I've personally seen it a couple of other times as well.
I'm a big fan of dual shear setups, just need to capture the top of the pivot somehow and keep that sucker from being able to oscillate. These are a couple of the pivots that I've repeatedly used over the years:
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