Here's exactly what happened:
I was playing the Egyptians, it's about 1985 AD. My workers have run out of stuff to do (it's late and they've railroaded, irrigated, and mined everything). When your workers are on automate and they've done all they can, they revert to manual mode after returning to your cities.
Well, next turn, Thebes produced some pollution. I hit automate on my workers and all 10 of them rushed to Thebes where they were executed by my mechanized infantry. WTF?!?!!?
They weren't captured, they were no enemies within my borders. Hell, I wasn't even at war with anyone. I guess Civilization is now factoring in depression or something because they off'd themselves quite nicely.
-Scott