I suspect it may have something to do with the BHPs Cannington mine being affected by the Qld floods, only a hunch. This being the case though, supply could be an ongoing problem for sometime!
"Located in north west Queensland, Cannington (BHP Billiton 100 per cent) is a fly-in fly-out mining and processing operation that has grown to become the world's largest and lowest cost producer of silver and lead."