Where did all the coupons come from?

Steam Christmas giveaways.

Santa's factory

coal farming with multiple accounts DX

heaven

Dreamland.

Walmart

The Steam marketing department - they figured it is a good way to make everyone buy more games. :)

During the holiday sale Steam users were given a series of objectives to complete within various games. Completing one of these objectives got you one of three things...

1. Coal (very common) - 7 coal could be turned into either a coupon or a gift (usually a coupon, it was random)
2. Coupon (uncommon)
3. A game (quite rare)

Since these things were also tradeable, some people were able to amass many coal/coupons.

Hope this answers your question :)