It appears when the Protagonist has climbed for too long, and has lost all Stamina. When this occurs, the actual Stamina bar itself starts dropping.
The player must keep left-clicking the mouse or tapping either the RT button on the Xbox 360 controller or R2 on the PlayStation 3 controller in order to remain climbing. Once the bar has been completely exhausted and the player has not reached stable ground to regain Stamina, the protagonist will die.
It can happen while running for an extended period of time, while climbing, and while walking through Dust. The Inhaler and the Adrenaline Syringe will replace the lost part of the bar, while a Piton can be used to avoid Extreme Effort during long climbs.