inexplicable defeat?

[LDiCesare]

Normally, there's a message explaining why the player 'died'.
The most likely is indeed dominion death: He no longer had dominion in any province.
Considering it requires some dedication to dom-kill someone, this can happen only if:
-he was playing EA or LA Mictlan and didn't do blood sacrifices.
-he had a very low dominion score (1 for instance) and was near someone with an even half-decent dominion score that flooded his(like 7 or more - I never took less than 5 in MP and even then only because I could blood-sacrifice to boost my own dominion).

[Shadowstar]

Yeah, it said he didn't rule a single province. That I knew was not true, and I could see that his dominion was even in some of my border provinces. I restored a backup and it seemed to fix the issue, we've gone a few turns now and it hasn't happened again. Might be some kind of weird bug but without knowing the cause, it's hard to replicate.

[Knai]

Dominion and province rule are two entirely different concepts. Dominion is represented by candles, representing the influence of the god within a region. If the god loses all influence, game over. Province control is a measure of which nation controls which province, and while there is a correlation between nation and dominion, particularly if temples are built, one can collapse without the other or grow without the other.