Obstructions include narrow nostrils, an overlong soft palate, enlarged and everted laryngeal saccules, malformed trachea. (dogbreedhealth.com)
These animals can have a variety of problems contributing to their breathing problems such as narrow nostrils, excessive soft tissue in the mouth and throat like the soft palate and relatively large tongues and they also tend to have very narrow tracheas (windpipes) compared to normal animals of a similar size. (vetsagainstbrachycephalism.com)