There has to be a few other things that trigger it beyond just distance or realm size.
I think the number of troops has an effect.
I did have a little chart with varying levels of troops, morale, loyalty & tax rate for Nbasah. I never tracked number of regions in a realm, but I wouldn't be surprised if that has an effect. It's long been lost to the mists of time and dead hard drives, so I can't quote any hard figures.
Try sending more troops up to Drinilla to arrest/hang those anarchists, that should sort them out! Long term, dropping more militia might be a good idea.