What Triggers Mega-Revolts in AGC-EEP?

[Seeker]

I was playing as Russia and annexed Mongolia. I was pretty religiously homogenous....but a little while later I got hit with this huge random event "Mass Religious Revolts"(there are too many minority religions in your empire!! it says)....and then I started to get all the Boris Godunov related crappy russian events...and the AGC-EEP Turks are huge and tough, better tech than me!!

First time I've had a real challenge from the AI, although they stupidly keep trying to siege the caucasus mountain provinces instead of landing in crimea and ukraine (they easily beat my navy)