Not pictured - the items on other side of the wall, the wall top covered in items, moat full of items, the horror on my face, the beer sprayed on my monitor, etc.
My theory is that when enough items are crushed together into an impossibly small space, they achieve a sort of critical mass and violently fly apart. My dwarves on the other hand insist I forgot to reset the atom smasher, but what do they know, they can't even match their socks.
Of interest, the furthest item flew ~19 squares and the spread pattern is a square, not a circle. The spread appears to be random and not weighted towards the center; there is a pile of 5 items a full 11 squares out yet many squares of single or no items right near the smasher. The items arc through the air; the bridge blocked most of the items flying to the southwest. Loincloths achieve remarkable distances. All animals and the one dwarf survived the blast with bruises, but the lever-puller, one of my starting seven, is quite shaken.
I'm toying with the idea of a goblinite atom-flinger landmine now, instead. It'll probably be even less lethal than a corpse shotgun, but the end goal here is really to humiliate invaders and not steal kills from my squads.