Communist AI is a pretty regular thing in most of my games, maybe because I usually don't play conquest, but go for a space race. In my experience, it's the expansionistic civs that are more prone to going commie in the late game, if they survive that is. They tech up for war (up to mech. inf. and nukes) and then they basically freeze research and max out on units. The question remains - why do they do the switch? Is it for economic gain (less overall corruption compared to monarchy or republic)? Or is it for pacifying the population (no war unhappiness + martial law)? Or is it both? Those are questions that darkpanda might be able to answer when he cracks the AI routines.