OK, there are some interesting theories going on Reddit, I will share the one I believe actually happened:
"May 2, 2013 â Empty Gox was sued by CoinLab for 75 million USD for breach of contract. For unknown reasons, Gox failed to fulfill obligations to provide server access, resulting in a startup-crushing financial liability for failing to deliver. (Rob Banagale) Interestingly, thereâs a massive selloff of 750,000 coins at an average price of about 100, totalling $75M, just following this event."
So in plain English, Mark sold customers' coins to pay off a debt last year. He probably thought he can make it back in commissions (there was a crazy volume of 1.1 million coins traded in the first week of May), and actually that wasn't a far fetched plan, but after May other exchanges started to take over the volume, and Gox lost lots of trust and business.
Then in August the US government sized 5 million dollars in their cash account further depleting their ability to pay withdrawals.
Then in December lots of Chinese customer started to pull money out following the Chinese ban and that started the bank run....
P.S.: The Coinlab case started in November, but it is possible he had to put up the money and it was frozen way before that.... That actually explain when he said "unavailable at this point".
https://bitcointalk.org/index.php?topic=477142.0
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Slight variation of this theory:
"Maybe it's as simple as Gox was low on funds, Mark thought he would roll the dice and take a chance he was selling near the height of a current bubble--versus having to close shop for certain if he did not do something to fix an incredible problem--and then the price actually exploded beyond any possibility that they could ever be bought back, in order to replace them."