  In post-inflation axion-like particle (ALP) models, a stable domain wall
network forms if the model's potential has multiple minima. This system must
annihilate before dominating the Universe's energy density, producing ALPs and
gravitational waves (a process we dub "catastrogenesis," or "creation via
annihilation"). We examine the possibility that the gravitational wave
background recently reported by NANOGrav is due to catastrogenesis. For the
case of ALP decay into two photons, we identify the region of ALP mass and
coupling, just outside current limits, compatible with the NANOGrav signal.
