Cooking off (or thermally induced firing) is unfired weapon ammunition exploding prematurely due to heat in the surrounding environment.
Inherent design flaws in early 17th century Swedish leather cannons led to the gun tube overheating which prematurely ignited the gunpowder, injuring the loader.
[3] Contrary to popular myth, this will not cause the machine gun to "runaway" at cyclic rate of fire (as compared to a slamfire) because each chambered round has to first be brought up to temperature.
[4] Apart from the possibility to cook off the heated propellant requires a special formula to allow for consistent muzzle velocity throughout all temperatures.
Because of the age and condition of the first two bombs, the fire safety crew was unable to cool them before they cooked off, which should have been possible for contemporary weapons with higher cook-off temperatures.
A different sort of cook-off event was the trigger for the 1969 explosion and fire aboard the USS Enterprise (CVN-65), which also involved a Zuni rocket.
During this event, the exhaust from an MD-3A "Huffer" Air Start Unit (ASU) overheated the warhead of a Zuni that was mounted on a parked aircraft, causing it to cook off.