As you conquer more territory in any theatre, all remaining factions there dislike you more and more. It's one of those things that makes sense on paper but when applied to all the minor powers in game results in a neverending series of free territory for you until you run out of idiot principalities that decided they could take on the local empire.

CA implemented a great little tooltip that shows this on the diplomacy panes, which is mentioned nowhere and only displays for a second or two thanks to yet another example of sloppy QA and no playtesting.