To my understanding it's clam as Lutpin said.
It was a coin that was created after Dooglus's justdice had to move away from BTC due to regulatory concerns. Basically you can "dig" BTC, LTC and DOGE addresses and get rewarded for having an amount of coin on your address at a certain block number.
Then afterwards, byteball did it. Then, everyone started to do it.
It's probably good to acknowledge Nxt as well. Nxt did an airdrop too for Ardor. 1:1 ratio of Nxt you own to Ardor. Now they're doing it again for Ignis "first child-chain of Ardor".