My friend and I work in a molecular biology lab, with regular access to a -80C lab freezer.  The other researchers' and students' sample tubes/vials are kept in boxes or sealed ziplock bags in the freezer.  Occasionally, after a period of storage, a random ziplock bag will be inflated almost to bursting point.  On speculating why this could be, I suggested that the water droplets in the air that is trapped in the bag will freeze rapidly and hence expand, and the pressure increase causes the bag to inflate.  My friend disagrees, reasoning that expansion of water droplets alone can't possibly account for the huge increase in volume inside the bag.  Who is right or, if neither, what is the correct explanation?