We provide a physical explanation for the enhancement of the low-energy electron production by sensitizing nanoparticles due to irradiation by fast ions. It is demonstrated that a significant increase in the number of emitted electrons arises from the collective electron excitations in the nanoparticle. We predict a new mechanism of the yield enhancement due to the plasmon excitations and quantitatively estimate its contribution to the electron production. Revealing the nanoscale mechanism of the electron yield enhancement, we provide an efficient tool for evaluating the yield of the emitted electron from various sensitizers. It is shown that the number of low-energy electrons generated by the gold and platinum nanoparticles of a given size exceeds that produced by the equivalent volume of water and by other metallic (e.g., gadolinium) nanoparticles by an order of magnitude. This observation emphasizes the sensitization effect of the noble-metal nanoparticles and endorses their application in novel technologies of cancer therapy with ionizing radiation.