Often that is the case yes. Once you're more than a few inches below the ground (especially if there is snow cover), the soil/roots will not be frozen. However water in the stem above ground, especially with snow packed tightly around the trunk, is frozen. Sap can't move up from the soil though the frozen section into the stem until it thaws out some. Generally that happens when the snow starts to melt back from around the base of the tree.