But remember that, in the vast majority of cases, your coins are converted to fiat currency on-the-fly, because fiat currency is what the merchant wants to receive. That is a sell order that pulls the value of Bitcoin down.
^^ This essentially is the real driver. Merchants are accepting Bitcoins through exchanges like coinbase. Rather than Bitcoins going directly to merchants, they're converted to fiat and the merchant is paid in fiat only.
It has been discussed in detail here:
https://bitcointalk.org/index.php?topic=407947.0