It shouldn't, if done right.  Withdrawing from a cold wallet should take some kind of conscious manual intervention.

If we assume that the wording in the crisis draft isn't literally accurate, then the following thread and article suggest one of the more believable theories I've seen about what might have happened:

https://bitcointalk.org/index.php?topic=489813.0
http://letstalkbitcoin.com/somethings-not-right-at-gox/

The TL;DR version:

A successful Transaction Malleability exploit emptied their withdrawal hot wallet for the first time in a long time.  Gox goes to refill the wallet during the initial BTC withdrawal freeze and ... uh oh... the cold wallet private key is lost/corrupted/not working.  It might have gone undetected because deposits and watching the wallet balance wouldn't evidence the broken private key.

(Edited for clarification)