  Particle-hole symmetry (PHS) of conductance into subgap states in
superconductors is a fundamental consequence of a noninteracting mean-field
theory of superconductivity. The breaking of this PHS has been attributed to a
noninteracting mechanism, i.e., quasiparticle poisoning (QP), a process
detrimental to the coherence of superconductor-based qubits.Here, we show that
the ubiquitous electron-boson interactions in superconductors can also break
the PHS of subgap conductances. We study the effect of such couplings on the
PHS of subgap conductances in superconductors using both the rate equation and
Keldysh formalism, which have different regimes of validity. In both regimes,
we found that such couplings give rise to a particle-hole $asymmetry$ in subgap
conductances which increases with increasing coupling strength, increasing
subgap-state particle-hole content imbalance and decreasing temperature. Our
proposed mechanism is general and applies even for experiments where the
subgap-conductance PHS breaking cannot be attributed to QP.
