As per an interpretation of the physiological effects,

"Excessive amounts of such fluid can impair breathing by limiting the expansion of the lungs during inhalation."

The normal expansion/re-bouding of the lungs is necessary for necessary natural physiological 'percussion/vibration' effect to allow natural mucous movements within the lungs. Ie. When the Type I Pneumocytes produce the mucous, the lungs require expansion for normal movement of mucous either into the lymph system, or coughed up. etc.