Thankfully, most medieval states didn't keep standing armies. Most of the states of Arunia don't either. Men with swords and training generally tend to grow bored when the fighting stops. This goes for mercenaries as well as for soldiers, the only difference being the terms of employment and feelings of permanent loyalty. Warriors, whether they be men or women, state or sellswords, do not deal well with downtime.
Endless examples assail us from history: rape, murder, theft, and brawling are all common side effects of having soldiers encamped for long periods. The ever-present danger of camp diseases increases, as does the tendency for defection and the accrual of camp-followers. Armies that are not on the march slowly go to seed, becoming mobile slums.
Mercenaries are the same way, though they may not present the same mass-problems that a huge army does (depending on their size). There's much adventure to be had with knights prepared for war who become restless and begin ravaging the countryside as though it were their right. The same goes for mercenaries, who are sometimes seeking to recoup lost pay and other times simply bored.
This is among the most morally ambiguous stuff you can throw at a party. After all, these are generally people and not monsters (though there's nothing stopping monstrous mercenary companies from doing the same). While some of them are likely just freebooters, killers, and general scum that avoided execution there are those who are men just like the PCs: adventuresome, filled with wanderlust, and unwilling to allow those of higher station to maltreat them.
Mercenaries-turned-bandits were a common problem in the middle ages, as were peasants raised for levies who then discovered their homes were gone or changed or found that once they had a sword in their hand they wouldn't have to be treated badly ever again. Indeed, the return of mass peasant levies who have been hardened by some conflict to their homeland has been the staging of many a rebellion and revolt: upon finding their situations less than equitable, they feel (and rightly) they are owed something better from those who used them so poorly.
The same problems can beset the men hired on or otherwise attached to an adventuring party. Without adequate checks and balances the adventurers may find that their massed horde of hirelings has been plundering the royal forests, stealing from inns and taverns, losing their minds over dice and tiles, and brawling amongst the city streets. Administration of sell-swords is a TOUGH business, and it can be made even tougher by the scrutiny of local magnates. After all, if you owned a fine little hamlet and several towns around it, would you want a gang of murders and tomb robbers and all their followers to take up in your local inn and perhaps set up a little camp nearby?
They will quickly deplete the region of game, buy up all the cheap food, and deflower all the farmer's daughters. No wonder adventurers have a bad reputation! They're wandering plagues with little concern beyond the immediate acquisition of gold! This is a great reason for landed military-service nobles in particular to look on adventurers with suspicion and mistrust.