JP Morgan is a bullion bank, that serves as counterparty for (ex-)owners (including governments and mines) that trade.
JP Morgan is a swap dealer, which acts in behalf of (ex-)owners that trade.
JP Morgan is a custodian, which stores for owners.
JP Morgans activity on the silver market, is nothing but a mirror on an underlying market.
At any price, every seller requires a buyer and vice versa.
Looking at JP Morgans trading activities and stock, is looking at (a part of) what people on the market generally do, and if its stock grows, regardless price, then that means more selling by mines/recyclers/whoevers stockpile/stack selling, and/or less demand from industrials/whoevers stockpile/stack buying.
JP Morgans stock reported on
http://www.cmegroup.com/delivery_reports/Silver_stocks.xls
is also just a part of a story that is bigger, the 6 Commodity Exchange warehouses hold now 175 Moz silver, JP Morgans share is 1/3.
For what it matters, JP Morgan does just the same on the silver market as on the dollar market: a bank accepts dollars from people that want someone to store>lend it out for them, regardless the dollars value relative to other values. It's a guaranteed counterparty. A buyer or seller that does what its customers want it to do, and it locks in its profit/hedges itself in any way, not only on the silver market, but cross-markets.