The rubber jungle is the term for the oxygen masks provided on jetliners in the event the cabin loses pressure. A pressure switch/sensor will cause them to deploy if the cabin altitude increases above a preset limit (between 10-14K feet).
There's one other time they get dropped and it's always an occasion for a red-face for pilots. If the landing is hard enough, some or all of them can be deployed by the G-force. Always get lots of passenger comments in this event! ;o)