Due to the depressurization that occurs in the cabin / hold of the aircraft.
In fact, airlines reduce the pressure by about -250mBar during air transportation. This is to minimize the cost of pressurizing the aircraft and to reduce mechanical stress on the aircraft. The same thing happens in the cabin where you travel.
Therefore, your container is under less pressure than its packing altitude, causing it to expand because the internal pressure balances the external pressure.
It is this principle of pressure difference that we use in our boxes. We can simulate air transport by putting a vacuum of -250mBar on your container and leaving it for a long time to simulate real air transport.