When an aircraft's cargo or passenger door fails to seal properly in mid-flight, the pressure imbalance within the interior and exterior of the craft can cause the cabin to essentially explode. Explosive decompression occurs when an aircraft is experiencing atmospheric decompression so fast that the air in an individual's lungs has no time to escape properly. This puts an enormous amount of stress on the human body, and can turn unsecured objects on the aircraft into a kind of explosive "shrapnel."
Aircraft fatigue over time can lead to explosive or rapid (slightly less dangerous) decompression. In these scenarios, an airplane begins to fall or break apart in mid-flight. The violence of decompression depends on the size of the aircraft, its altitude and the size of the damaged area on the aircraft exterior.
Explosions caused by manmade weaponry like bombs or damaged aircraft machinery like engines can present significant and immediate damage to the aircraft, causing explosive decompression to occur and, if most of the craft remains intact, oxygen masks to drop.
While far less dramatic, turbulence and rough landings can cause the panels that hold an aircraft's oxygen masks in place to become unfixed, allowing the masks to to drop as if cabin depressurization had occurred.
If an aircraft's crew forgets to engage the craft's pressurization system, a slow but life-threatening form of gradual decompression can occur as the plane ascends, causing the human body to be deprived of its oxygen at such a slow pace that discovery of the problem sometimes does not occur until it is almost too late.
