This is because of elastic fibers in the pulmonary parenchyma which tend to collapse the lung. A particular type of inspiration, the sigh, is most important in preventing the lung from becoming atelectatic. A sigh transiently inflates the lung to total lung capacity, preventing passive atelectasis. Gravity adds an additional force to the normal elastic forces of the lung which tends to collapse the lung. Unless you are a bat, this tends to be the posterior and inferior aspects of the lung.
Cicatricial atelectasis is pulmonary collapse due to parenchymal or pleural scarring.
The most common cause of this is right middle lobe syndrome.