In the normal lung:
Compression of the airways by intra-thoracic pressure during maximal expiration.
During normal inspiration and expiration the intra-pleural always remains more negative than the alveolar/airway pressure
During forced expiration both the intra-pleural and alveolar pressure increase. However, because of the pressure drop across the airway the pressure in the airway becomes less than the intra-pleural pressure, tending to close the airway.