There will be a big increase in the airflow through a building if there is somewhere for it to go. To get the curtains to flap upward like Kate described I'd imagine there needed to be two windows open or a window and an external door open. But It is hard to tell now, but in Kate's words she checked the sliding door and that was shut. But she didn't check to see if the window in the main bedroom was also open.
It goes in one side and out the other all the time quite naturally because buildings are not airtight. What is the cause of the increase in flowrate to which you refer? Temperature differential? pressure differential? artificial means?.
Just start with a few basic premises:
1. Air moves from high pressure areas to low pressure areas.
2. No building is air tight so there will always be infiltration/ exfiltration through natural leakage paths.
3. The curtains moved in a specific direction so you can reasonably define where HP and LP were at the time the curtains moved.
Work from that and try to think in a larger domain size than a single room or apartment.
I doubt any conclusive answer will be reached though. Is it important in the long run? it has no bearing on what colour curtains Matt saw and is that relevant to anything?