So they apparently drop when others craft badges. The rate is dependent on your level, the rate is clearly very low...
I have got 1 booster pack so far. This trading card business has kind of lost it for me since the summer sale. All you do is play your games for a few hours to get the drops, then wait for an eventual booster pack
And once you craft a badge, you don't get more drops for the first level, you just have to wait for ages...







