deSitter
Regarding the AD, what exactly is a severed frame? TIA
Aluminum fuselage construction involves a series of hoop like frames that give shape to the fuselage. If fatigue cracking initiates on the inner or outer face of the hoop, it can progress until it cracks through all portions of the hoop, finally completely severing the hoop's continuity. At that point, all pressurization loads are being entirely handled by the skin in the area of the severed hoop (frame).
One can envision the entire fuselage tube bending down aft of the wing during turbulence and during firm landings. The tube bends by flattening its outside radius in the area of the bend. The frames resist this flattening which is how they become exposed to fatigue.
Note: Those who design aircraft structure are welcome to improve on this description--Not my area of specialty.