There is some speculation that MtGox does not have adequate BTC on hand to cover a significant volume of bitcoin wallet withdrawals.
By halting withdrawals it buys them time to (among other things) either; a.) increase their bitcoin balance, b.) come up with another excuse not to give customers access to their bitcoins, or c.) hope that this 2-day delay will somehow slow the bleeding once they re-enable withdrawals on Monday.
d.) Buy back the bitcoins at a now much lower price.
Is it really out of the realm of possibilities that before when the price was 15-20% higher then other exchanges that they didn't sell XBT they never actually owned! Once customers wanted those XBT, they simply stopped paying out and are now actively buying them all back at a 30%+ discount. Give it some time with gox price much lower then other exchanges and watch how magically the "technical issues" will resolve themselves.