It has to do with the conditions you're playing in and how gameplay is progressing against or in your favor; Mega events are there to stir things up when any particular "situation" triggers it.
Example; Jagged Knife -- this normally happens when everything is neutrally boring in the galaxy and everyone (AIS & you, again) is safely tucked in their own territories and enjoying a status-quo.
It's also a roll of the dice; what are the odds in Casino craps that i'd throw an eleven twice in a row? The parameters & calculations are MUCH more complex in GC2.
50 turns is a very low figure on a Gigantic map too, btw.