At start of check, window is open and door is in almost open position, there is airflow in through window, through door, and out kitchen vent chimney (gravity and venturi effects suck air up the vent chimney). The air pressure in the kitchen is almost the same as the pressure in the bedroom because the air is continuously flowing to make the two pressures almost the same.
Now almost close the door. The same volume of air per second still flows bedroom to kitchen through the doorway but through a much smaller gap so the air velocity is much greater and produces a large force on the door at an angle almost 90 degrees making it slam to fully closed.
The pressure in the bedroom is now much higher than in the kitchen because the airflow which tries to make them the same has been blocked.
Now open the door. Briefly there will be an airflow at a much larger velocity than before, to get rid of that large pressure difference, of enough velocity to blow the curtains in. This is when the curtains blow in. Then within seconds the airflow will reduce to the slower rate same as at beginning of check, and the curtains will stop blowing in.
The physics is all good and the witness is telling the truth IMO.