What happens when relaxation of the diaphragm and intercostal muscles increases intrapulmonic intrathoracic pressure?

When the diaphragm and intercostal muscles relax, intrapulmonic (within the lungs) and intrathoracic (within the chest cavity) pressure both decrease, not increase.

During inhalation, the diaphragm contracts and moves downward, and the intercostal muscles contract, elevating the rib cage. These actions increase the volume of the thoracic cavity, lowering both intrapulmonic and intrathoracic pressures below atmospheric pressure. This pressure difference causes air to flow into the lungs.

Conversely, during exhalation, the diaphragm relaxes and moves upward, and the intercostal muscles relax, lowering the rib cage. These actions decrease the volume of the thoracic cavity, increasing both intrapulmonic and intrathoracic pressures above atmospheric pressure. This pressure difference causes air to flow out of the lungs.