Apple crisps feature a delicious play of tender cooked apples, caramelized sugars, and crisp topping. This is not a situation where you can loosely top your dessert with a loose layer of plastic wrap and call it a day. Without airtight storage, freezer burn, a common culprit for dry or discolored frozen foods, can occur. When exposed to the cold air of the freezer, moisture from the dessert "sweats" out and tiny ice crystals form on the surface. Once thawed, icy spots can result in a dry apple crisp with a soggy exterior.
Most foods have a significant amount of moisture - ice cream's water content ranges from 55% to 64%. That moisture freezes into ice in the freezer, but then begins to melt whenever the freezer is opened and lets warmer, drier air in. These air and temperature fluctuations lead to a cycle of melting and re-freezing, which leads to that layer of ice crystals forming on top of ice cream;