Two different people reported within two days that minor faction armies/unique spawns sieged a city in 2.04. As it is not intended, this could have been caused by the join/fighgt support routine. I know this feature was bugged and cities/castles joined a fight if you engaged close enough to that location. Whereas this has been fixed to apply correctly for the player it could still be bugged for the AI, namely, the special armies and unique spawns.
I can´t test this ingame though as it is beyond my skill and it´s just a nasty little suspicion, and I reckon this hasn´t changed in 2.5 as well due to the recent notifaction on this board.
Apart from looking like a cosmetic bug, this one could become rather lethal if the minor army/unique spawn actually TAKES the city or castle and the game tries to decide what to do now.