Might be the effect of big merchants dumping coin as well...
big merchants dont hold coin .. they use Bitstamp or CoinBase to convert bitcoin payments to USD instantly. Even Overstock only keeps 10% of the bitcoin it receives as Bitcoin, cashing out the rest as USD
That is what I am saying... the insta-dumping is providing more liquidity in the market and hence the fall. If u notice carefully, u'll see that when bitstamp went down for a brief period, market was rising. This is because the liquidity went sudden low for a short span of time.