Wagner are actualy the coup masters, that was like their main job. Wagner's bread and butter before the war was getting control of small countries and especially gold and diamond mines and other valuable resources in Africa. Maybe Prygozhyn reckons russia is about as weak as one of those countries now and he can take it. He has some popular support. Sure russia has the nukes, but they cant use them to stop a coup, are they gonna nuke themselves?