Daniel Mark Harrison, writing for CoinDesk:

Rising volumes and declining prices do not naturally come together since when an asset is being more heavily sold than it is bought, buyers typically stay away from the market until there are no more sellers left.

While illegal in many regulated markets, price support is not prohibited in bitcoin’s case since the asset is widely unregulated. If carried out for only short spaces of time the practice can be cathartic in that it introduces needed liquidity to exchange floors.

Interesting read. There are lots of different theories on what's driving the price down in spite of higher volumes on the exchanges and expanding adoption.