It appears the heavy hitters are pulling out of BTC and going into Alts such as PPC and NMC and others as of late...
The more simple answer is there are more sellers (mostly miners) than buyers (retail or institutional).
3600 bitcoins are mined everyday
But why Miner selling on lose, I do not think they are even ROI on mining on this price.
Miners are not necessarily selling at a loss. You can't look at retail mining costs to assess large, well-funded mining operations. Likely, many large operations have sources for cheap production of up-to-date ASICs and access to cheap electricity.
Still, at this point, the only option to maintain operations is to pay your bills in fiat. So..... it should always be assumed that anyone invested in a mining operation may have reason to sell, regardless of price.