There are two types of events in the system
1) random-events that can happen at any point in the game as long as the proper conditions exist determined by "die rolls". These all have a "mean time to happen" value that basically reflects how often the event will happen, each day a die is rolled for each event, if the roll is low enough that event happens, roll again the next day, if the roll is low enough it happens again, etc.
2) triggered events-as Grommile said, these events are triggered by something specific happening in the game rather than happening at any time, however, the most common "triggered events" are those triggered by pulses such as the random event pulse (approximately every two years), the ideas event pulse (triggers events related to idea groups, I believe on a 5 year pulse), religious events pulse (also 5 years), and possibly more. There are also events that trigger other events (Burgundian Inheritance event triggers events for France and whichever nation gets the Imperial holdings for example) or, actions in the game can trigger events
Events triggered by a pulse work differently than the truly random events. If the stars aligned, every random event in the game could fire on consecutive days. With the pulse-triggered events, when the pulse occurs all eligible events get thrown into a basket similar to the NBA draft lottery with each event getting a number of "balls" based on how many positive factors it has (if any are specified in the event) then one ball gets picked and that event happens and that is it for those events until the next pulse occurs. Looking at the event you listed I would guess it is one of the random event pulse possibilities but you would have to look through the files to figure out for sure
I also spend my days making games so forum time is always going to be somewhat limited from actual developers.
