The key is that I was on the bank of a stream. As I recall, some heavy-ish rains had eroded the bank somewhat and left the roots semi-exposed on the lee side. At least, that's what my forensic analysis of the tree concluded when I could look at things the next morning. Had I been setting up in the daylight I suspect it would have been very, very obvious that the tree was unsuitable.
My take-away lesson is to play it extra safe if setting up after dark: don't pick a site for it's great morning views if you can't inspect it fully. When setting up in the dark, pick safety over scenery.
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