They are capable of attacking without declaring war, though they're not Hidden Nationality. I'm not certain at the moment as this machine doesn't have Civ4 installed, but I believe they use AlwaysHostile? Although, now that I think about it, the cities they were stationed in might have also had a Ring of Warding up. However, none of the stuck units were summons or summoners. I don't recall having any issues with normal Hidden Nationality units causing WoCs, even when a Ring of Warding is involved, so chances are this particular lockup is due to AlwaysHostile or whatever flag/attribute is used on Dragon Fanatics and Wyvern Riders.