Pressurization system operation or systems failure. The pressurization system on an airliner is relatively simple: it comprises one or two "outflow valves" with discs that can be modulated from a full-open to full-closed position. As air enters the airplane from the air conditioning system, the pressurization system measures the pressure in the cabin and modules the valves to a correct value to maintain an optimum pressure for the phase of flight.
When the valves get stuck, or the crew fails to open the engine bleeds following a max-performance takeoff, a subtle pressurization failure can occur. We distinguish this type of failure from "rapid decompression", which usually occurs when a window blows out or a cargo door opens in-flight. See "Systems - Pressurization - Bleeds-off Pressurization Emergency".
There are also blow-out panels for over- and under-pressurization situations in order to preserve the integrity of the pressure hull.