I heard some rumor that it's a safeguard to prevent people from corpsing to enemies in dungeons.
If that's a thing, it works very inconsistently (if at all). I've been instantly corpsed (and even badly impaired) in battles before, sometimes by taking up to 5 hits' worth of overkill in the round I was knocked out.
Something I learned pretty quickly: Don't cast fireballs at (or otherwise aggro) entire groups of archers all at once unless you can soak up the ensuing arrow storm with damage resistances.
If you take massive damage, you roll fortitude vs damage or become Badly Impaired. However if you go down with -1, it does seem to behave differently. For example I noticed that magic tends to affect downed people inconsitently; sometimes evards will corpse them sometimes not, fireballs usually do no damage, etc.
Nope. Not what I'm talking about.
My characters have on several occasions been badly wounded or outright downed by volleys of arrows from bandit or skeleton archers, typically when they suddenly entered combat against a whole group (either by being detected while sneaking/ninjalooting, or by casting an AoE spell and drawing a ton of attention to themselves). In many cases, it got to the point where I got brought to <10 HP, before taking another half a dozen arrows and immediately landing in the Near-Ethereal without ever getting the chance to stabilize. I
have occasionally seen myself take absurd amounts of lethal damage (also distributed across multiple physical attacks in a round) without instantly corpsing, but certainly not enough to attribute it to some kind of protective functionality instead of a bug.
But yeah, most
spells seem to avoid corpsing unconscious PCs.