Karpeles says that any Bitcoin he personally owns was on Mt. Gox, meaning he will lose money if the exchange fails. In response to a question about how much he personally stands to lose, Karpeles writes, “well, technically speaking it's not 'lost' just yet, just temporarily unavailable.”
Do you see what this means? At first it seems to make no sense...
How can Karpeles have his personal bitcoin stored in the exchange, YET it is temporarily unavailable?
If he has bitcoin stored in the exchange which he owns, he surely has the private keys to it, right? right! But then it would be AVAILABLE to him.
So we conclude he does not have private keys to that bitcoin on the exchange. So that bitcoin is NOT real. It's fake bitcoin he created simply by entering numbers into the database, which would explain the sharp decline to 91.5 USD as pointed out above.
Noone sane would have sold at 91.5 USD when other exchanges were at 500-650.
He created BTC out of thin air and pushed the price down, in order to get more fools to buy that low priced goxBTC via the builder for real BTC he would then keep dumping.
edit: Also to get more fools to deposit more funds into MtGox for him to grab more real USD
Those buying the fake BTC were trying to multiply it with the sharp up and downs in that period and resell to others trying to do the same in this ponzi which just ended when MtGox shut down and the last one to hold those fake goxBTC got burned.