The expansion is probably due to ice and not air. Vegetables contain water, and water expands when frozen. This expansion can cause the cell walls to break. Any slight thawing causes the water to leak out of the cells. When refrozen, the water is now on the outside of the cells, and refreezes as a block of ice. Slight thawing may occur from a power outage, storing your vegetables in the door (the warmest part of the freezer), or constant opening of your freezer door. The vegetables are probably safe to eat, though the quality may have deteriorated.