The Mega Events I believe have some sort of factors involved in determining if they occur, so I've heard. Some people, depending on their playstyle have said certain Mega Events happen quite often, while others with different playstyles seem to rarely see them, but see others. I for one, have yet to encounter the peacekeeper mega event, yet get dread lord event probably once every 5 games.
Also, I believe (Sole Soul said this) that setting Extreme Planet Quality occurance (during game setup) to lower levels, makes events happen less frequently. So having extreme planets on abundant should give alot of events, whereas having extreme planets on rare will give hardly any events. I have somewhat verified this, because I hardly got ANY events at all (when I used to play with Extreme PQ's on rare), now I have set them to be in the middle and have since gotten events happening more frequently.
Sounds like some type of bug to me though, since events should not be effected by extreme PQ setting.