Clues to the cause of recurrent fluid can be obtained by careful examination of the fluid, in the laboratory. These tests include measurement of cell content, levels of various biological markers, protein content etc. Heart failure can result in fluid in just 1 lung or in both, as can clots to the lung.
Fluid in just 1 lung, also called unilateral effusion, post-operatively is common and may be related to the manipulation and irritation/inflammation of tissue that occurs with all surgery. If a cause cannot be found, ultimately, your father’s doctors can make the pleural surfaces adhere to prevent fluid re-accumulation.
This is in all likelihood a temporary condition. The good news is that your father felt fine, after the fluid was removed, suggesting that the surgery was a success.