I think by announcing this at night, you minimize the risk to innocent bystanders going about their day to day. No matter when they decided to announce, the media would have found a way to say they were wrong. It is a lose lose for the authorities. The only people to blame for the violence and looting are the criminals and looters.
Mark Levin said something to the same effect - The announcement was made at night to give everyone who wanted to a chance to get home without traffic jams.