It also can happen when your fuel regulator goes snafu and you put 10lbs of fuel pressure to your float valve lifting Holley. You end up with 1 or 2 - 1/8" streams of fuel running down yer engine's gullet. That'll wash the snot out of a set of rings.. not to mention destroying your engine
oil's lubricity too..
or.... simply a piece of trash under one of the said valve seats..
or... a dead/unfiring plug
or... no compression in a/several cylinders, i.e. unburnt fuel,
Typically ring wash IS NOT caused by a carb's rich mixture.