In frost heave conditions, a steady source of water to the earth below the foundation is subject to freezing temperatures, and ice lenses (ice wedges) are able to form and grow, thus pushing the foundation up and out of the ground. The severity of this effect is determined by the ability of the soil to retain moisture and the climate. In certain conditions, this force is strong enough to lift the foundations of a deck causing the deck to separate from the house.
Even if the results are not this catastrophic, the prolonged effect of this rise and fall of the deck will take its toll by weakening the structure and creating an uneven surface. Because of this, you need to install the foundation for your deck at a suitable depth below the frost line.
Frost jacking is a similar phenomenon that can also act to lift frost footings. Frost jacking occurs when the upper part of the buried pier is grabbed by frozen soil and pushed upwards. Adding a flared base to the footing will help hold it in place. The colder the temperatures in your region, the deeper you will have to dig. You can find your local footing depth requirement by asking your local building inspections department.