This is the results of tubeless inner-liner failure.
The failure normally is caused by a hard shock to that area; debris left on the road, etc. The hard shock causes the tubeless liner to split slightly and the inner air pressure leaks out thru the other layers of rubber but doesn't get thru the outer sidewall layer.
This situation normally progresses very slowly, and is another good reason to check tires before and during each trip. The bubble starts very small, gets larder and larger, and will eventually pop, exposing carcass ply(ies).