Decompression is an unplanned drop in the pressure of a sealed system, such as an aircraft cabin, and typically results from human error, material fatigue, engineering failure, or impact, causing a pressure vessel to vent into its lower-pressure surroundings or fail to pressurize at all.
However, in this case the flight took off this morning from SIN and 45 minutes later the masks started to drop. It was a false alarm as the pressure in the cabin was normal. The captain turned the aircraft back and got all paxs to leave the plane.
Pictures were taken after all paxs have disembarked.