...the actual landing itself, and not the descent from altitude, it's probably the aircraft depressurizing.
Every transport I know of has a "squat" switch of some type or another that is closed when the weight of aircraft settles on the main gear struts. Think of the main gear struts as big shock absorbers with an electrical switch on the top/bottom halves that connects when it's compressed.
The reason for this system (some call it an air/ground sensor, or other names) is so that the aircraft "knows" absolutely positively when it's on the grouind, and when it's in the air. Why? There are some aircraft systems you want to work *only* when on the ground and *never* while you're in the air, and vice versa. Examples are thrust reversers, ground spoilers (only on the ground, never in the air), and on the flip side, landing gear (always in the air, never on the ground).
Another thing tied to the system is the pressuration system's outflow valve, which opens onces you're on the ground, allowing the cabin pressure to match the outside airport pressure. If this didn't occur (automatically, to avoid "human error") and the aircraft bellied in with fuselage intact, nobody onboard could escape (a possible fire?) since the still-pressurized cabin would be pressing down on the plug-type entry doors, preventing them from opening. A bunch of folks on a Saudi L-1011 met their doom this way after a successful emergency landing years ago. Nobody could evacuate since the cabin was still pressurized, and everyone burned with the aircraft.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.