I have seen a lot of valid explanations on here actually. I tend to think its due to some of the larger merchants who have begun to accept btc, like Dell and overstock. As people spend btc at those merchants and they instantly convert it to usd, its essentially like a lot of coins being sold. more selling drives price down, and not enough (hardly any) new buyers/new money coming in.
Yet who is going to spend BTC at Dell or Overstock if they already have $Fiat?
Seems plausible that all these sales are being driven by Bitcoin whales and professional miners who are dumping. You have to wait until these Bitcoin whales / miners run out of BTC so the price can recover.
It could very well take years for Bitcoin to go back to $1000 (due to all the overnight millionaires created overnight who'll need to be liquidated) but then Bitcoin isn't the only game in town, it could be replaced by an alternate in those years of recovery.