First of all, I doubt that secretion in normal healthy persons is nil, secretion of fluids and transport of mucus is needed to keep the lungs clean.
The reason I think, for the increase in secretion is the ineffectiveness of this transport due to obstructive or restrictive lung diseases. Perhaps the mucus cells in the lung parenchym will react to the increase in particles in the lung by increasing secretion in the hope of cleaning the lungs via the mucus transport system.
Keep in mind, this is just my hypothesis!