My theory about the Mtgox fiasco is this:

Karpeles used mtgox's clients (fiat /btc) deposits to manipulate the market. He probably was doing that with the help of bots. For example, if he wanted to increase the price he was "borrowing" fiat from mtgox's depositors and with that fiat he was buying btc, creating an uptrend. As more mtgox's clients were buying bitcoins there was a moment when the clients who had fiat in their accounts and wanted to buy bitcoins couldn't  because Karpeles had already bought bitcoins with their money. So, at that point Karpeles had to sell the bitcoins that he had already bought in order to give the Mtgox's clients fiat to buy also. ( this also explains the huge lag, he needed time to do this).

So this strategy should had given Karpeles a lot of profit because he was buying cheap bitcoins with borrowed money (which caused an uptrend) and when he was forced to sell bitcoins (which were more expensive) to cover his clients needs, he was selling less, so he had profit in bitcoins.
But this is not true because there is also one more parameter, the liquidity. When he was forced to liquidate bitcoins he might had to sell more bitcoins than he had bought if liquidity was very low.  So the extra bitcoins had to be created from thin air which caused a btc fractional reserve. Same logic applies if he wanted to create a downtrend with a fiat fractional reserve as a consequence.