Since the cruising altitude can be up to 41,000 feet, modern commercial airplanes use pressurized cabins for the passengers to breathe. The compressor normally pressurizes the cabin to the air pressure level of 6000~8000 feet high. However, when the sensor detects the air pressure is lower than this level, the oxygen mask will drop.
There can be different reasons, such as a broken window in the cabin or cockpit, seal failure of the fuselage, failure of the pressurizer. Besides breathing difficulty when the cabin losing air pressure, you may also experience weightlessness after the oxygen mask drop. The reason is the oxygen carried by a civil airplane can only sustain 15 minutes, the pilot must descend as quickly as possible to 10,000 feet before running out of oxygen.
Updated on SEP 20, 2020