When gases in containers are heated, their molecules increase in average speed. This means that they exert a greater force when they collide with the container walls, and also collide with the walls more frequently. The gas is therefore under greater pressure when its temperature is higher.
Molecules of a gas move randomly. In a sealed container they exert a force when they collide with the container walls and this applies a pressure to the container. If the cylinders heat up enough, their pressure will increase and they will explode.
