How to have towns collapse on their own?

Back then I was playing Teutonic campaign for Medieval II Total War. There is a Norwegian faction which is unplayable and rigged to not be able to train units. I waited and did not attempt to conquer the last town. At the same time the game assigned me a merchant mission. Eventually the town collapsed and fell into the hands of rebels faction. Naturally a notice appeared saying I no longer had to complete the merchant mission.

So is there any particular way to incite rebellions in rival towns? It seemed interesting and coincidental. My guess is moving your army near an enemy settlement. This might increase unrest I don't remember exactly.

Move spies into the city, and use your assassins to destroy buildings that create Public Order.