What causes an AI to surrender?

There's been some discussion on these forums of the possible factors, or lack thereof, influencing who an AI surrenders to, but as far as I know there's never been a discussion as to what causes an AI to surrender in the first place. In some cases, a race surrenders while still maintaining a strong military and economy, while in other cases they don't surrender at all, but fight to 'the last man' - quite a pain on gigantic all-abundant maps.

Does anyone know what factors might go into convincing an AI it's all over?