  We analyze the quantum quench dynamics in the formation of a phase-biased
superconducting nanojunction. We find that in the absence of an external
relaxation mechanism and for very general conditions the system gets trapped in
a metastable state, corresponding to a non-equilibrium population of the
Andreev bound states. The use of the time-dependent Full Counting Statistics
(FCS) analysis allows us to extract information on the asymptotic population of
even and odd manybody states, demonstrating that a universal behavior,
dependent only on the Andreev states energy, is reached in the quantum point
contact limit. These results shed light on recent experimental observations on
quasiparticle trapping in superconducting atomic contacts.
