Answer: The primary cause of air leakage in any system is the pressure difference between the system and its surroundings. Whenever the system pressure is below the surrounding pressure and there are openings, the air will be pushed inside the system through the openings because of the pressure differences.
As stated in the question, when the condenser is operated below the atmospheric conditions the possibility of air leaking inside increases.
The more the pressure difference between the system and surroundings the more air leakage would happen.