Price falling because Supply > Demand.
The real question is WHY Supply > Demand.

It's unlikely for Demand to drop drastically in short period of time.
Therefore, the main suspect is Supply.
What could be a reason for Supply to increase drastically?
GenTarkin's hypothesis of miners needing to sell more coins to cover costs is plausible, but unlikely.
Miners are not stupid to shoot themselves in the foot by over-saturating the market.
More likely is that somebody is dumping a great number coins (possibly in distress).

P.S. Also, some large miners quitting is a good thing for me - they quit, I get more coins.
I don't care what they worth in fiat right now, I hold them for a long run.
P.P.S. To all 51%-attack-phobists, even if 90% of miners drop out, this attack is still very improbable.