PD growth?

[Arryn]

I've noticed that several of my provinces are increasing their PD levels by 1 periodically, without an event doing so (the events, when you get them, increase by 5, 10, etc.). It's only in certain provinces, and it's not hidden sites because in at least one of these auto-increasing PD provinces I've done an Acashic Record and there's nothing there that accounts for it (or that says anything to the effect). It's welcome that my loving subjects wish to improve their defenses, but I am curious as to the underlying trigger for this, and why only in some provinces, and never in others (when I do get the free increases, it's always the same few provinces).