The air pressure is also higher at colder temperatures so if the fire is underground it'd have more forces working in its favor to cycle fresh air to it and the

The air pressure is also higher at colder temperatures so if the fire is underground it'd have more forces working in its favor to cycle fresh air to it and the relative temperature difference between the freshly heated and exhausted air and the fresh air would be wider causing there to be more turbulence cycling the air around.