I never paid attention to these doors on MiG-21's before, but after watching a number of videos of 21s on the ground, it looks like the door position on your excellent model has a bug.
From the videos, the purpose of the doors seems to be to allow sufficient airflow into the engine in conditions when ram air pressure into the main intake is low. The doors seem to follow these basic rules:
1. If the engine is off, the doors are closed.
2. If the engine is on, and the plane is not moving, the doors are open.
3. If the engine is on, and the plane is moving quickly, the doors are closed.
If you looks at the following pictures of 21s, you can see the door position in shutdown 21s, as well as 21s that are taxiing:
http://www.hottail.nl/airforces/czech/Mig-21.html