Everyone selling big amounts of goxBTC at the moment is arbitraging. You can easily make 50% in just a few hours by doing this:

1) Deposit into Gox via OKPay - it takes a few hours to clear.
2) Buy goxBTC
3) Sell goxBTC for "real" BTC at 0.6ish while the real price is 0.4

Example: at $250/goxBTC, for $10,000 you get 40 goxBTC, you can sell them for 24 realBTC. 24 realBTC on stamp at $625 equal $15,000.

4) You can send the real bitcoins back to OKPay and get ~$15,000.
5) You now have $15,000 to buy more goxBTC with.

Rinse and repeat and it's a money printing machine, a pretty fast and incredibly profitable arbitrage.

Prices are going down because there's a lot of competition between people doing this, so the market corrects itself.