It’s really frustrating when you’re protecting a caravan and a band of raiders attacks, then they sit on defense. There should be some sort of requirement to make the attackers actually attack. If they attack the caravan i’m protecting, they shouldn’t be able to sit there forever and force me and mine to charge into their shield wall in order to progress the game.
I’ll bring up another video game here for an idea of a solution: Age of Wonders III.
In Age of Wonders III, sitting around as the attacker waiting for the defender to come to you will count as a retreat for you if you wait too many turns. Once you retreat, you get penalties on the worldmap (namely, you lost a battle, so your faction’s morale is reduced, and your army’s morale is reduced, and you lose all remaining movement points, etc.).
While all those mechanics won’t transfer over to Battle Brothers well, I could see the enemy being forced to go on the offensive if they’re the attacker after X number of turns.
