Why does the diesel engine develop an airlock after the vehicle is allowed to sit for long time?

A diesel engine can develop an airlock when the vehicle sits for a long time due to the potential for fuel to drain back into the tank, especially if the fuel lines or injectors leak. This results in air entering the fuel system, creating a void that prevents proper fuel flow to the engine. Additionally, the fuel can become contaminated or degrade over time, further complicating the starting process. Consequently, the engine may struggle to start or run smoothly until the air is purged from the system.