Stack Effect and an open top floor window
I think you're asking where air would come from in a modern, well-sealed home, in order to replace that which escapes through an open window. The answer is simply that there's no such thing as a "sealed" home. Even brand new, ultra-efficient homes have countless tiny leaks. Door seals. HVAC penetrations. Recessed light fixtures. Gaps in vapor barriers. They happen. 

Older homes were so leaky that it was almost as though a window was open at all times. Modern homes reduce that leakage dramatically, but you're still probably talking the equivalent of a 3-4" pipe through the wall. By opening a window, the convection and escape create low pressure that dramatically increases the airflow through those leaks, probably on the order of ten times as much. 

There's also the natural mixing that occurs, which has probably as much effect or more as the overall leakage volume.