Gatherix
Death by Darkly
It all started with those TF2 account farms. More people farmed, causing more metal being produced, resulting in inflation of metal.
People have been doing that since forever, and the inflation would have happened without that anyway. The big catalysts in the drop of value in metal were the update that made all items from the Mann Co. Store craftable and the Steam Market, both of which were semi-recently added by Valve.