The Death Weapon is a pseudo-weapon that is triggered when a specific unit is destroyed. The few units that cause this kind of damage are described as Volatile. These units are experimentals, mass fabricators and power generators.
It is worth noting that Death Weapons apply their full damage to all targets within their radius. By comparison, Area of Effect weapons like Artillery and Strategic Missiles are divided amongst all targets they hit. In practice, this means that when your T2 Mass Fabricator dies, it doesn't divide the damage of its death weapon among the other T2 Mass Fabricators around it. Instead, it will destroy them all. Conversely, this also means that that suicide CZAR you sent against an enemy will do 10,000 damage to every single unit it land on, killing practically everything and bypassing shields (since the Death Weapon isn't triggered until the CZAR hits the ground, under your enemy's shields.) Comparatively, a strategic missile may do a lot of damage, but if you have multiple T3 shields soaking it up, your hardiest T3 structures (and potentially even your ACU) can survive it because all that nuke damage is divided out amongst the targets. In effect, a Death Weapon can be a nightmare for you defensively, or a nightmare for your opponent if used creatively, and you should always remember this no matter which end of the bargain you find yourself on.