  On the analogy with the Lamb shift, we study the vacuum effect that proton's
electric field interacts with virtual particles in the vacuum. We find a
possible quantum instability that triggered by an external force, proton's
electric field interacting with virtual particles spontaneously induces a
quantum force that back reacts on the proton in the direction of the external
trigger force. Such a quantum-induced force accelerates the proton runaway, by
gaining the zero-point energy from the vacuum (~10^{-5} eV/cm). This effect
possibly accounts for the mysterious origin and spectrum of ultra high-energy
cosmic ray (UHECR) events above 10^{20}eV, and explains the puzzle why the GZK
cutoff is absent. The candidates of these events could be primary protons from
the early Universe.
