When breathing for life, inhalation is active involving many muscles and exhalation is passive.
There are muscles between the ribs, and over the shoulders, some extending from the back of the head.
When the diaphragm flattens, these muscles combine to expand the ribcage. Even muscles low down in the back may be called upon to stabilize the spinal column.
Think of the massive timing process that the brain must habitually orchestrate to repeatedly coordinate all these muscles.
In contrast to inhalation which is so active and complicated, exhalation is the easiest thing we ever do (unless we have emphysema).
To breath out we simply relax and the elastic nature of the diaphragm returns it to its normal dome shape (helped by the push of the viscera which were compressed during inhalation). The chest cavity collapses helped by gravity. All this forces the air out. It is a totally passive process.