Two ab initio methods have been employed to calculate the dynamical potential energy surfaces (PES's) for the excited (B-1(2) Or (1)A') and the ground ((1)A(1) or (1)A') states in the Mg(3s3p(1)P(1),)-H-2 reaction. The obtained PES's information reveals that the production of MgH in the (2) Sigma(+) state, as Mg(P-1(1)) approaches H-2 in a bent configuration, involves a nonadiabatic transition. The MgH2 intermediate around the surface crossing then elicits two distinct reaction pathways. In the first one, the bent intermediate, affected by a strong anisotropy of the interaction potential, decomposes via a Linear HMgH geometry. The resulting MgH is anticipated to populate in the quantum states of rotational and vibrational excitation. In contrast, the second pathway produces MgH in the low rotational and vibrational states, as a result of the intermediate decomposition along the stretching coordinate of the Mg-H elongation. These two tracks may account for the previous experimental findings for the MgH distribution, which the impulsive model has failed to comprehend. By far, different interpretations have been proposed especially for the low-N MgH product. The supply of a detailed PES's information in this work helps to clarify the ambiguity. It is also conducive to an interpretation of the isotope and temperature effects on the product rotational distribution.