Why do some patients, (icu, ventilated), get subcutaneous air which makes a crackle sound when you touch the area affected?

Sub Q air: This can occur because air is being forced into the lungs under pressure and air can escape from the lung into the pleural space and also dissect up the center of the chest (mediastinum) and under the skin.

Barotrauma: That is called subcutaneous emphysema. The shearing force and pressure and volume of the ventilator causes area of tissues in the lung and air spaces to burst or pop and the air seeps into surrounding tissues. Make sure these are monitored and ICU aware that it is not real lung collapse.