I keep meaning to post this quote from Labarge's Henry V about garrisons in Normandy during the last years of Henry V.
"[The English soldiers] had to be paid with some regularity if they were to be kept in service, and restrained from harassing the local citizens whose loyalty was sufficiently tenuous in any case. The problem was acute. The soldiers, unless actively engaged in campaigning were often bored, and frequently without money. They naturally turned to lawlessness and many preyed upon the inhabitants, since the king's strict regulations [. . .] made desertion and escape to England very difficult. They thus helped to aggravate the problem of brigandage in Normandy [. . . ]"
and skipping ahead a little:
"Desperate bands of dispossessed men hid in the woods and caves, controlled many of the roads, and attacked solitary travellers. Their membership sprang almost completely from the lower peasant class whose security and livelihood were most cruelly affected. The nobles had too much to lose to join this kind of resistance."
During this time, it goes on to say, the number of wolves dramatically increased - so much so that when bandits were killed (or captured and executed) they were buried, to avoid drawing wolves to their hanging corpses.
I like the image that brings - dispossessed people, those with something to lose more willing to cooperate with the invaders, etc. Good gaming material, even leaving aside those opportunists who turn to banditry only when it comes along. Fans of Seven Samurai will know what I mean here - a soldier's misfortune is a peasant's potential gain.
By the way, if you read a lot of Medieval European history books at once, it's amazing how often it comes to "stop having wars for a while and come back home and suppress all the bandits that arose" seems to come up as an activity. It gives you a feeling of barely-suppressed disorder in the homeland, fractious nobles and unreliable allies wanting to get theirs before you get yours, and rulers scraping up armies to go abroad and fight with borrowed money and limited time. It's a far cry from organized professional military forces with logistical trains, solid tax bases to support them, and wars of national concern. Nevermind the convoluted nature of fighting when you've got two rival claimants to a throne, both with strong legal cases to make for it. Or those odd cases in English/French history when the English king rules one part of France in fief from the French King, but also is fighting the French King in his role as English King. Messy stuff. Pick a side, or get trampled underfoot and turn to banditry? Or both?