More food was being generated by the site than is being used up.
This eventually devalues food.
So for instance if you used to sell a fairy coin for 30k

and there is a lot more

floating around the site, now you can ask for 90k instead.
Over time this builds up into a big problem.
Firn created the Unique Knuffel Auction as a way to reduce the amount of food on the site. You saw how much food it cost to win, right?
All that food is now gone from the KofK economy. It is out of the market.
It will take a long time for that much food to be created again.
So it is a "drain", because the food is 'going down the drain' -it is going away, and the overall amount of food is lowered.
Make sense?