Mt.Gox is one of the biggest disasters in Bitcoin history and the story is not yet complete.
Based on the available evidence, it appears that Gox lost a fairly modest amount of money (in terms of USD) in 2011 that compromises a decent bulk of the missing money. They were subsequently hacked a number of times that have been publicly reported, during which they likely lost some amount of money.
I am not entirely sure why the losses compounded over time, maybe Gox was not a profitable operation, and they took customer money to fund the losses.
Security did not seem to be a strength of Gox's. If they had been honest about the loss in 2011, it would likely have been not a very big deal, but Gox would have closed and some other exchange would have taken the spot of having the majority of trading volume.
I heard the bitcoin dump is not the first time on this June but also several weeks ago when Bitcoin is heading to reach USD 10,000. On that time, actually bitcoin is heading to exceed USD 10,000 but this Mt Gox releasing their bitcoin to the market in big numbers then impacted to price decreasing until less than USD 8,000. Now, they releasing other bitcoin to press bitcoin price to be lower than USD 7,000.
The Gox trustee did in fact sell a lot of coins on the market starting from the mid teen's down to near the February low.
In January the price was about $17k, and in Feb the low as $6k, and quickly bounced back to just under $12k. The trustee stopped selling before the price hit $6k, and the price has since declined back down to $6k multiple times, and breaching that low this past weekend.