This is caused by several reasons, in some cases, it’s a normal phoenomenon, otherwise, that’s a problem need to be fixed.
Check if the package is intact, whether ther’s damage to the tray or sealing film, or bag sealing quality that allows exit of atmosphere and air inlet.
Check the packaging material permeability. Atmosphere changing might be caused by the low barrier sealing film. The use of low barrier or high barrier sealing film is different for different food products.
The vacuum process wasn’t done perfectly, so some air is trapped in the food texture and then released into the atmosphere.
The atmosphere and food product interacts by absorbing, dissolving or producing gas.