They have been given out as special compensation before I believe. Well before my time on this account so I'm not sure exactly what warranted them, though I believe I saw them posted about on ATA's social media a long while back.
It was one of the first reward boxes. Ata did a social media giveaway and a small amount of players won the boxes.
And of course ATA are still running these giveaways hence we still have the keys in the marketplace of course