Observations have recently been made of ion cyclotron emission (ICE) that originates from the core plasma in the DIII-D and ASDEX-Upgrade tokamaks. In some of these cases, the ICE spectral peaks correspond to the local cyclotron harmonic frequencies of fusion-born ions close to the magnetic axis. This is in contrast to the hitherto usual spatial localisation of the ICE source to the outer midplane edge in tokamak and stellarator plasmas. Here we show that a possible emission mechanism for core ICE in ASDEX-Upgrade deuterium plasmas can arise from the rapid onset and rise of local fusion reactivity. This would give rise to a transiently highly non-Maxwellian population of fusion-born protons near their birth energy, prior to collisional slowing-down on longer timescales. Such populations are often liable to fast radiative relaxation under the magnetoacoustic cyclotron instability which underpins ICE, depending also on bulk plasma parameter values. We therefore perform first principles computations of the self-consistent collective relaxation of such a population, using a particle-in-cell code, for plasma parameters appropriate to the ASDEX-Upgrade core. The resulting simulated ICE spectra include a strong peak at the proton cyclotron frequency that corresponds well to the observations.