Spontaneous optical emission of graphene irradiated by intense single-cycle terahertz pulses was studied experimentally and explained theoretically. We found that the emitted photons are mainly polarized normally to the electric field of the terahertz pulse, which proves that the terahertz field not only heats the electrons, but also creates a strongly nonequilibrium momentum distribution. The comparison of the measured optical spectrum and polarization anisotropy with the results of numerical modeling allowed us to estimate the momentum isotropization time for electrons in graphene to be ∼25 fs and roughly reconstruct the time evolution of the distribution function in k space.