Tom is right about why this happens. One way you can usually minimize or stop this happening is to close your doors slowly; gradually. That usually will allow the fire to begin drawing it's combustion air from other points and maintain the convected flow up the flue.
Most door systems do have some venting above, below, or both. If the venting is adjustable, you may need to be sure it's open before closing the glass doors.
Also, being a newer unit, it's possible you do have a fresh air intake which may be closed; check for a small lever or door usually located low on one sidewall inside the firebox itself.