There need to be the right conditions. You need at least two civilizations that are developed enough to produce nuclear units. They need to be close enough to be at odds to each other. Neither must have the United Nations wonder. One must slightly be larger than the other and want to dominate the other.
My theory is when one group is strongly overpowered they launch their nuclear arsenal just like they do at you when they feel it is the last resort or you have a Cold War scenario.
I have seen it on most of my strange world scenarios I do. I know during my South American Isolationist Empire game I witnessed a three way fight for Africa.
Russians had Asia, Europe, and Egypt. Zulus had central and southern Africa. Americans had a city in Morocco and another in west Africa.
Imagine this scenario...
The Russians are dominating the world. Their last enemy in the connected continents of Europe, Asia, and Africa are the Zulus. If they took Africa I would have them crossing from Africa to South America instead of perpetually fighting the Zulus. The Americans with only a footprint in Africa brought 20+ bombers over from the Americas and laid everyone in the Sahara to waste.
I stopped the flood of Russians into Africa by blocking the Suez with diplomats, leaving only two cities in Egypt under Russian control. They still attacked the Zulus with remaining forces. And, out of nowhere, the Russians nuked the Zulus.
I moved my precious Settler NONE units I keep across the ocean and landed them near the bombed out city to clean up the pollution.
As I just started the cleaning Russians nuked that city again and two others. Zulus shot them back! Zulus even hit the Americans with one in Africa.
I literally bought every transport I could and in one turn had 40 units in 5 transports across the sea. I needed to eradicate the Zulus and use my United Nations Wonder to force peace.
Two nuclear winter turns hit. I had a handful of Zulus cities, but I was down to a little over 15 Armors out of my 40. Russians were hit with a nuke by the Americans! I tried to cross the desert to destroy their ports, but those bombers ended my army in the Sahara before I managed to hit their cities only a couple times.
That scenario ended in the Nuclear Winter.