DIAGNOSIS: The gasket magnets have lost their strength or the door is misaligned, breaking the magnetic seal. This is a known issue with heavy doors over time.
FIX: First, check the alignment. Loosen the hinge screws on the top of the door that's popping open, gently lift the door to reseat it, and retighten the screws. Test the seal.
IF THAT DIDN'T WORK: The door gasket itself is faulty. You'll need to order a replacement gasket for your specific model, carefully peel off the old one, and press the new one into the channel all the way around the door.
HEADS UP: This wastes a massive amount of energy. Until you fix it, the fridge and freezer will run constantly, so tackle it quickly to avoid a huge electricity bill.