How did almost 30% of the keys(~350k) disappear in Jan 16?

[abhijeet]

Okay, this might make me look silly but I'm quite new to the whole tf2 thing. I was casually checking the key prices today when the price graph caught my attention. According to the graph, around 1100k keys were in existence on 4th jan and that number plummeted to 800k on 25th jan. What caused this sudden drop? How did almost 30% keys vanished in the span of 20-30 days? Where did they all go?

[Mengh.]

Backpack.tf cleared up old / mismatched items in their database of items when swapping their database provider to Elasticsearch.

 

http://steamcommunity.com/groups/meetthestats#announcements/detail/991156744036068555

http://steamcommunity.com/groups/meetthestats#announcements/detail/988906472315598415