Wagner's chief (aka the only remotely competent military over there EDIT: yes they also do lots of war crimes) is moving against Putin. This being the same "elite military" branch that Putin basically sent out to die in Ukraine, so, um, I imagine they have grievances.
EDIT: The people replying to this are much smarter about tank-and-gun stuff than I am, so for better context of what's tanking and gunning over there please see replies, heh