In the past when I've been wiped by barbs (not uncommon lol) it's swapped me to another civ which I assume is a feature. That way you don't lose all the time you invested into the game. I would doubt a capital would have issues with revolutions, unless you're treating your people really poorly. Could be sneaky barbs. Might also be an AI strike team, if you didn't have much of a garrison. Idk if the AI would do that though.