An optical laser works by bouncing photons back and forth between two mirrors, one totally reflective and one partially reflective.
If the lasing material (usually a crystal) has the right properties, photons scattering off the atoms in the crystal will excite cascades of more photons, all in lockstep. Eventually the beam will escape through the partially reflective mirror.
One kind of sorcerer's apprentice mode involving bounce messages can produce closely analogous results, with a cascade of messages escaping to flood nearby systems.
By mid-1993 there had been at least two publicised incidents of this kind.