Sometimes AI just get a tad too busy with real life stuff, walk their AI dog, throw their AI ball around with the AI kids you know, pepi and his little daughter Butch Anderson. Then there's the other AI, you all know him.. Lets just say he's got some dents to work out of his car...
Basically yeah their economy has been shattered, they've been diplomatically embarassed, or cultural influence is so close to teetering that they only had a few minutes left... I honestly wish you could turn off their ability to surrender.