Overstock.com - They accept Bitcoin, they probably did so many transactions today, but all of those coins were dumped on the market. Thus driving the price down. Imagine how many bitcoins they made today, but they didn't hold onto any of them, instead they just dumped all those coins. Anyone who paid in Bitcoin on Overstock today, pretty much just helped lower the price.
reddit is like

but anyway maybe these giants selling huge chunks of coins into the market will cause a few dips here and there (opportunities to buy?), but i think the price will adjust to these high volumes that will be going on daily.. and if these merchants spread awareness and bring more money into bitcoin the price will increase naturally
PS: I am not an economics pro or anything, but it just came to my mind and wanted to share it here.