I find the simpest way to get these guys appearing is to take a settlement and then destroy all the buildings you can so the settle can only produce peasant and then withdraw and allow it to revolt... Hey Presto, super peasants...

I think the AI gets a certain amount to spend on the troops generated in a revolt and the amount is depedant on the size of the settlement... And when you have a large city but you have destroyed the barracks and stuff the only way it can send the cash is on super peasants... What do you think???