Didn't get it from any source, but according to me it's just because free electrons in the d orbital and one excited electron in the p orbital. Sigma bonds will be formed in lower energy orbitals hence one s and 3d. And access to me the three orbitals for Sigma bond must be dxy,dyz,dxz cuz approach of electrons in Sigma bond is not along axis. So pi must be formed by the ones mentioned in question. Just wanted to confirm because it's a very rare case where these d orbitals are used for pi bonding in rest of common cases in general dxy,dyz and dxz are used.