Looking over my Mt Gox trade log, I do not see any single large transaction that caused the move. Rather there are many small, almost random sized transactions on the way down and on the way partly back up. Here is one of the larger ones that occurred shortly after the 16.65 level was breached ...
[mtgoxUSD 1309472003463345 2011-06-30T17:13:23.000-05:00 [email protected]]
It would be a chance for certain market makers subject to the .3 % commission rate to make a profitable trade - given how relatively large the spike down was. I suppose that once the swing began, existing bid limit orders were cancelled and moved lower thus drawing the price downward in the face of continuing market sales orders. I do yet track Mt Gox open order issuance, matching and cancellation, but once that happens it will be easier to explain this sort of behavior - but only the point of ignoring open orders in the dark pool.
Modern trade execution prefers to break up a single large order into seemingly unrelated smaller chunks. Even if executed from the dark pool to hide the order, trades become public knowledge once matched. Breaking up a such a trade is a tactic to out-game front-running algorithms.