[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).