Well, my boyfriend, his brother, and I just discovered something pretty cool.
All three of us were playing Hearthstone games on the same router. I had just finished whooping the ass of my boyfriend in a game, when he suddenly got the cardback. It’s supposed to be for scheduled Hearthstone events only, and we were very confused why only he had received it.
We tested a few things and found a trend. If you disable and reenable “Challenging nearby players” in the options menu, challenge one of your two friends using the same Wifi, and make sure to purposely lose that game, you’ll be granted the cardback.