Diffuse relic neutrinos with light mass are transparent to ultra-high-energy (UHE) neutrinos at thousands of EeV, which are born through the photoproduction of pions by UHE protons on relic 2.73 K blackbody radiation (BBR), and originate in active galactic nuclei (AGNs) at cosmic distances. However, these UHE ν's may interact with others (mainly the heaviest: ν_μr, ν_τr, and respective antineutrinos) that are clustered into hot dark matter (HDM) galactic halos. UHE photons or protons, secondaries of ν-ν_r scattering, might be the final observed signatures of such high-energy chain reactions, and may be responsible for the highest energy extragalactic cosmic-ray (CR) events. Here we consider the conversion efficiency, ramifications, and energetics of these chain reactions for the 1991 October CR event at 320 EeV observed by the Fly's Eye detector in Utah. These quantities seem to be compatible with the distance, direction, and power (observed at MeV gamma energies) of the Seyfert galaxy MCG 8-11-11. The ν-ν_r interaction probability is favored by at least 3 orders of magnitude over a direct ν scattering onto Earth's atmosphere. Therefore, it may better explain the extragalactic origin of the puzzling 320 EeV event, while offering indirect evidence of a hot dark Galactic halo of light neutrinos (i.e., m_ν~tens of eV), probably of τ flavor.