What about this scenario?
Someone opens a maximum 10:1 short at 5.9ish (executed internally?). Said person has massive amounts of BitCoin, and since he got an optimum price on his short, he sells hard. He leaves a large enough ask wall to get a good liquidation price, then closes his short position, opens long, pulls asks and begins buying at cheaper rate. Essentially double dipping. This would be easy for someone with the funds.
In this situation, bitcoinica would not cause much effect, but the person selling on Gox to make the most of his Bitcoinica position would be the reason.