Every season, each player who hits rank 20 or higher gets a fancy new card back, and each player who did not get there does not. The vast majority of card backs are, presently, only available once.
Earlier today, Blizzard announced that it is giving us a gift! That is, they are giving us another shot at the Gift Exchange tavern brawl and, as a prize for winning a brawl this week, the Winter Veil Wreath card back.
Players who already have the card back will receive the traditional classic pack upon winning, so everybody wins.
Official announcement here.