Postoperative atelectasis: This is the most common cause of minor bibasal subsegmental atelectasis, especially in patients who have recently undergone surgery. Atelectasis in this context is often due to shallow breathing and retention of secretions.

Pulmonary embolism with infarction: While not the most common cause, pulmonary embolism can lead to atelectasis, especially if there is associated pulmonary infarction. The bibasal distribution might not be typical, but it's a consideration, especially with risk factors.
Chronic obstructive pulmonary disease (COPD) exacerbation: Patients with COPD can develop atelectasis due to airway obstruction and secretions. The bibasal distribution can be seen, especially if there's a component of bronchiectasis.
Pneumonia: Community-acquired or hospital-acquired pneumonia can cause atelectasis, particularly if the infection involves the lower lobes.

Pulmonary thromboembolism without infarction: Even without infarction, a pulmonary embolism can cause atelectasis due to vascular obstruction. It's crucial to consider this diagnosis due to its high mortality if untreated.
Malignancy: Lung cancer or metastatic disease can cause atelectasis, either by direct invasion of the airways or by causing lymphadenopathy that compresses airways.

Rheumatologic diseases (e.g., rheumatoid arthritis, lupus): These conditions can cause pulmonary manifestations, including atelectasis, due to inflammation or fibrosis.
Cystic fibrosis: While more commonly associated with upper lobe disease, cystic fibrosis can lead to atelectasis in any part of the lung due to chronic infection and airway obstruction.
Lymphangitic carcinomatosis: This rare condition involves the spread of cancer to the lymphatic vessels of the lung, which can cause atelectasis among other symptoms.