Unless the bug being fixed was some sort of critical security failure that enabled people to game the exchange or manipulate balances. I am guessing this is more of a screw-up by amateurs than a 'take it all down until this hole is patched' situation, but there is a slim possibility that keeping it up was a bigger risk than the reputation hit of extended downtime.
Looks like these are announced improvements to the Exchange engine. Really look forward to a post-mortem on this issue.
Curious if they had issues with database migrations or such, that required recovery from backup in case of errors. Plus, for sure there were bugs in the code itself.