We present a model to obtain the parameters of collisional resonances (quasibound states) in collinear A + BC reactions. The resonances are interpreted as quasibound states trapped in wells of adiabatic potential curves as functions of the minimum-energy-path coordinate. The adiabatic partial widths are obtained from semiclassical tunneling calculations. The nonadiabatic partial widths are found by using a Feshbach golden-rule formula with the decay mediated by an internal centrifugal interaction proportional to the curvature of the reaction path. Exact two-dimensional scattering calculations are then carried out and fit to the isolated-narrow-resonance formula to obtain the accurate partial widths. The model results agree well with the accurate values.