Atomic is simply the adjective form of Atom.
Atomic often refers to the energy contained inside of a object that is unimaginable small. How small? you ask. Watch this:
So, when people talk about going atomic to describe how angry a person gets or crazily out of control someone gets, they are referring to the energy of space that is insanely densely packed with matter.
Maybe, going atomic is like too many densely packed negative thoughts in your brain at once. Thus, people like atoms, can have any little thing that disturbs it's stability, causing an explosion.
