This thread is for people who, like me, were not completely aware of the "Troublesome Regions script". The following informations come from the campaign_script.txt located in data\world\maps\campaign\imperial_campaign (line 12574 to 12935).
The idea of this script is to represent the renowned instability of several regions by "randomly generating additional Rebel presence". In-game, it means that every winter there are 5% chance that a Rebel army (8 units) spawns in the region accompanied by an important rise of the unrest (between 10 and 20) in the settlement.
Note from QuintusSertorius : the presence of the Rebel stack in the province will add unrest as well as the scripted addition, based on the size of the stack and how long they've been active in the province.
Map of the Troublesome Regions
List of the Troublesome Regions