  Even though the Standard Model (SM) has achieved great success, its
application to the field of low energies still lacks solid foundation due to
our limited knowledge on non-perturbative QCD. Practically, all theoretical
calculations of the hadronic transition matrix elements are based various
phenomenological models. There indeed exist some anomalies in the field which
are waiting for interpretations. The goal of this work is trying to solve one
of the anomalies: the discrepancy between the theoretical prediction on the
sign of the up-down asymmetry parameter of $\Lambda_c\to\Sigma\pi$ and the
experimental measurement. In the literatures several authors calculated the
rate and determined the asymmetry parameter within various schemes, but there
exist obvious loopholes in those adopted scenarios. To solve the discrepancy
between theory and data, we suggest that not only the direct transition process
contributes to the observed $\Lambda_c\to\Sigma\pi$, but also other portals
such as $\Lambda_c\to \Lambda\rho$ also play a substantial role via an
isospin-conserving re-scattering $\Lambda\rho\to\Sigma\pi$. Taking into account
of the effects induced by the final state interaction, we re-evaluate the
relevant quantities. Our numerical results indicate that the new theoretical
prediction based on this scenario involving an interference between the direct
transition of $\Lambda_c\to\Sigma\pi$ and the portal
$\Lambda_c\to\Lambda\rho\to\Sigma\pi$ can make both the decay rate and sign of
the asymmetry parameter to be consistent with data.
