Why did multiple stacks die on a nuke hit?

I saw my enemy had a Nuclear Bomber set to attack my stack of 12 arty. So I separated 10 of the arty and had them leave the city, while the 2 arty (the stack that the nuke would attack) stayed put, to minimize losses. But once the nuke hit, I saw that both stacks had been destroyed. In the newspaper, it said that the enemy nuke killed both of the stacks. How is this possible? Is it a bug?