Propane tanks are designed to vent under certain conditions. This is good because the tank itself then does not explode. This is bad because the vented propane must be dissipated before it encounters a spark.
A cover on the back of a pickup must be vented if you store a portable propane tank in it. If there is no air passing through, and the tank vents you just created a bomb ready to explode. Terrorists use this property to create car bombs.