All systems should have a separate shut-off valve upstream of the regulator, in this case, at the tank. Whenever the forge is not in use, the cylinder should be shut off at the source. Never rely on the regulator to guarantee that the flow of gas has stopped. Petcock valves are basically bursting disks designed to vent excess pressure, particularly when the tank is being filled. However, they can burst on their own, the most common cause resulting from laying a full tank on its side, which increases the pressure on the valve.
Excess vibration can also be a contributing factor if the tank is full.
Heat is rarely a factor, since propane is about 300 degrees colder than gases such as acetylene. Propane cylinders can leak and it is best to assume it does leak. Murphy s Law should and must be applied to anything with explosive characteristics. This means that it can run along the ground to a source of ignition, then flashback. It can also drain into a sewer, or, for that matter, the lowest point of your vehicle, wherever that might be - which means it might be beyond your refractory senses.