I'm quite amazed where these huge sellers get their coins from.
Remember those 850.000 bitcoins "lost" on the MtGox? Somebody has the private keys of those BTC. Ask yourself what would you do if that was you. Would you hold all of them, or would you convert some to the fiat?
For some time, the current number has been 650k MTGOX coins "lost"... b/c recall that 200k were recovered.
Yes, there remains considerable uncertainty about who holds those coins and when they got possession of them... whether it is the US Govt, or some central bank, or some common hacker thief, or Karpeles and friends, or just some random dude(s)... also, whether they are white hat or black hat and when did they get a hold of those coins.. could have been as early as 2011 when Gox was having some hacker attacks and could have been as late as late 2013 - if you go with Karpeles et. als. apparent and implausible version of events.