A primary pneumothorax is one that occurs without an apparent cause and in the absence of significant lung disease, while a secondary pneumothorax occurs in the presence of existing lung pathology. In a minority of cases, the amount of air in the chest increases markedly when a one-way valve is formed by an area of damaged tissue, leading to a tension pneumothorax. This condition is a medical emergency that can cause steadily worsening oxygen shortage and low blood pressure. Unless reversed by effective treatment, these sequelae can progress and cause death.
Although this is a human, you can see a left sided collapsed lung and how it is atrophied <shrunken> these can be caused without apparent damage or disease in the lung.
Here is a collapsed lung in a right side lung, the arrow indicates the lung and you can see how much smaller it is compared to the left lung.
Oxygen deprivation can lead to all kinds of problems from stroke to death. As oxygen is deprived, carbon dioxide builds up in the blood and there is little to no exchange of oxygen at the celluar levels. The body cannot dispense of the carbon dioxide properly and ataxia can set in causing neurological problems. Oxygen deprivation can be from as little as 4 minutes before the body starts shutting down and once that happens without immediate intervention you can have clinical death. If oxygen is not restored within 10 minutes you can have biological death. The time frame varies but is never more, in rare cases of 45 minutes with intervention. The heart, brain and kidneys need a constant supply of oxygen in order for them to function, when deprived they start shutting down quickly.
In the case of a beardie, laying him on his back causes the body to flatten and gravity takes over. This flattening regardless of connective tissue, causes the organs to spread out, and there isn't much room with all the organs in a small compact body. Lungs and the heart are contained within the chest or thoraxic cavity held within the ribs. Visceral organs are also contained within the lower ribs to include the stomach, liver, pancreas, kidneys. Intestines and other organs are contained within the abdominal cavity. As these organs are flattened out they spread out putting pressure on the lungs and heart. This increases the pressure in the chest cavity. With the spreading of the organs within the ribs this impedes the ribs from functioning in aiding the inhalation and exhalation of air. The resulting increase of pressure in the chest cavity causes the lung to force the air out of itself and in turn collapses. The lung itself is a spongy tissue full of air sacs <aveoli> bronchi that are branched off the bronchial tubes. Immediate intervention is needed to release the pressure within the chest cavity for the lung to re-inflate. Oxygen deprivation can also lead to hypoxia or cerebral hypoxia. Hypoxia occurs when organs and tissues are deprived of oxygen and they start to shut down. The brain needs a constant supply of oxygen and if deprived goes into cerebral hypoxia where the brain starts dying. Brain cells are extremely sensitive to a lack of oxygen. Some brain cells start dying less than 5 minutes after their oxygen supply disappears. As a result, brain hypoxia can rapidly cause severe brain damage or death.