Over 89,000 BTC was exchanged for ETC (Ether Classic) over the last 24 hours on Poloniex . That's $53 Million at a $600 USD/BTC. I speculate this sudden windfall for ETC holders is partially being liquidated into fiat. Thus the drop in the price of BTC.
Some articles are blaming the meeting between bitcoin classic developers and miners for the downward pressure. Any drop in price can be used by the "blocksize increase" camp to further their agenda.
I think BTC has become the defacto settlement layer for all the other blockchains. BTC is the on and off ramp into the fiat world, whether that is USD, EUR or GBP. Therefore a massive buy of ETC will put downward pressure on BTC.
There is also a lag between exchanging in and out of fiat because of the sometimes week long delay with ACH banks transfers. We can see the elasticity in the BTC price being reflected by this lag.
The BTC is dead meme needs to be put to rest once and for all. I see the shift in mainstream articles focusing on "let's legislate" BTC away from "let's outlaw" BTC articles. Therefore this drop is just another buy and HOLD opportunity.