  The decay of massive particles during inflation generates characteristic
signals in the squeezed limit of the primordial bispectrum. These signals are
in particular distinctive in the regime of the quasi-single field inflation,
where particles are oscillating with masses comparable to the Hubble scale. We
apply the investigation to a class of scalar particles that experience a
so-called waterfall phase transition in the isocurvature direction driven by
the symmetry-breaking mechanism based on the hybrid inflation scenario. With a
time-varying mass, a novel shape of oscillatory bispectrum is presented as the
signature of a waterfall phase transition during inflation.
