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Long, long time ago, around 2014 iirc. But it was 14% of the entire supply, so an extremely generous airdrop. I would rather not parrot my own guess as fact, so just clarifying this is my guess, but I assume they wanted to build a passionate/dedicated community for governance purposes. I mean building good will through an airdrop is always possible, but it can lead to pump & dumps (e.g. Ethereum Dark; not that this coin is particularly comparable to Decred, but the p&d kind). If you have a real purpose behind it, you can get a lot further with an airdrop, and in their case they always put governance top of the heap.