Secularly growing loop corrections in scalar wave background

We consider two--dimensional Yukawa theory in the scalar wave background $\phi(t-x)$. If one takes as initial state in such a background the scalar vacuum corresponding to $\phi=0$, then loop corrections to a certain part of the Keldysh propagator, corresponding to the anomalous expectation value, grow with time. That is a signal to the fact that under the kick of the $\phi(t-x)$ wave the scalar field rolls down the effective potential from the $\phi=0$ position to the proper ground state. We show the evidence supporting these observations.