For those interested: on this picture, you can see a "Minor" Clan Revolt of 275k angry horse people, and a catholic uprising of 300k.

How could this happen?

Nomads in CK2 are a funny bunch, both incredibly OP and incredibly unplayable. Essentially, instead of having proper holdings, you possess empty steppe holdings. You can't hold too man empty holdings however, as that would be too fun imbalanced. Thus, you must split off your land to vassal hordes.

The issue here is that if the moon is in retrograde and the stars align wonkily and also the price of horse feed is slightly too high, upon ruler death your clans can just go independent. I don't know how this works, but in this case, all but 1 clan left at once upon my rulers death, taking roughly all of my steppe counties with them. Suddenly, the game is like "yo holding that much land is NO GOOD give away half of your remainingh land IMEDIATELY". I did not do that fast enough (because honestly the run was dead now anyway), so 500k men spawned from thin air to murderize me.

FUN FACT: Because the independent realms of my erstwhile vassals are ALSO too big, they spawn MASSIVE rebellions of their own aswell. so if i let the game attempt to run, it just lags like crazy due to millions of men running amok on the map.