The reason behind the payout is because of how the ETH/ETC split went down back in the day, resulting in them getting a huge extra amount of ETC. They didn't spend it, and they actually went ahead and passed along the earnings to the holders. I wouldn't say it is a totally selfless move in that giveaways are a great way to earn permanent public goodwill. But it also made buying DigixDao anywhere in the $60-70 range a really unique buying opportunity (I got in at $70).
As far as when you could expect the block payment, they had announced it was at block 3,800,000. You can find out about more coin giveaways on bitcointalk forums for other coins, but I don't think there will be anything quite like this again.
awesome thanks