In the movie The Last Starfighter, the Death Blossom is a "weapon of last resort" that is used when completely surrounded by all of your enemies.
In the Iraq War, Death Blossom was military slang to describe when the Iraqi military would receive contact and start firing wildly in all directions.
One again Q has a masterful multiple meaning crumb.
My take is that much like in the Last Starfighter, the enemies are being drawn in close, to the point where they think they are about to land a killing blow, before the Death Blossom of DECLAS takes out the enemies that are EVERYWHERE.