I just noticed something using the CRPViewer: The AI abandoned a town because it built a worker or settler (probably worker) with no surplus food. First I thought that some other civ destroyed the town (very early in the game), but the replay shows that it didn't happen that way. So the only explanation I think is abandonment. The town was next to some luxuries so the AI was probably eager to get a worker out and stumbled on its own feet, unless someone has another explanation. I have noticed that the computer sometimes chooses a zero food tile to work as default when the tile has plenty of gold or shields.