Well, generally speaking, "It's random." Certain situations can favor one or the other. For example, being at war increases the likelihood of a Stability Decrease event firing. (In my Lombardy learning AAR, I twice had situations where that event fired less than a month before the end of a war. Ouch.)
On the whole, they follow the pattern of "Good situations make good things happen. Bad situations make bad things happen." It would take too long to list all of the modifiers to the Improve Stability event(s), but here are a few just as examples of what makes the event fire faster/more often:
Having lots of gold (thresholds at 100, 1000, and 5000); having lots of prestige (at 100, 500, 2000); having lots of piety (at 0, 100, 500, 2000); having Royal Prerogative or Popular Law (Feudal Contract has the opposite effect, by the way); the fewer vassals you have, the more often it fires.
If you want to see the whole nine yards, crack open your CK folder and go to DB/Events/Stability_Events.txt. Once you understand how the event notation works, reading some of the event texts can shed a lot of light on how the game works. Of course, this takes a fair amount of time and patience, but then again, so does the game.
