So, thanks to the advice of Max
here, I finally managed to actually explore a dark fortress for the first time without being overwhelmed by lag. But something pretty damn strange happened while I was exploring around.
A small group of human, elves and dwarves were milling around the site, slaughtering the inhabiting goblins. At first it seemed like a pretty standard invasion, but I found the supposed leader of this army. She was a elven lady, and a member of the very goblin civ she was attacking. I thought it strange that she was a lady, since I presumed that goblin civs only had demon lord nobles (unless said demon was killed). I started looking around legends mode, and found that the position of Lord of this particular dark fortress had been created by some other goblin nearly two hundred years earlier. He lived as the "lord" for nearly fifty years before being killed by his demon master. It wasn't until I loaded up the save into legends viewer did the phrase "insurrection" popup for this goblin lord.
So, I guess my question is - was this really a goblin insurrection? I knew that other civs had the occasional insurrection, but I never thought a goblin would attempt to overthrow their demon masters. But since this does seem to be the case, anyone have a guess as to why the original goblin was able to live pretty long as a rebellious lord before the demon got to him?