There have been intensive studies on the newly discovered phenomena called aggregation induced emission (AIE), in contrast to the conventional aggregation quenching. Through combined quantum mechanics and molecular mechanics computations, we have investigated the aggregation effects on the excited state decays, both via radiative and nonradiative routes, for pyrazine derivatives 2,3-dicyano-5,6-diphenylpyrazine (DCDPP) and 2,3-dicyanopyrazino phenanthrene (DCPP) in condensed phase. We show that for DCDPP there appear AIE for all the temperature, because the phenyl ring torsional motions in gas phase can efficiently dissipate the electronic excited state energy, and get hindered in aggregate; while for its "locked"-phenyl counterpart, DCPP, theoretical calculation can only give the normal aggregation quenching. These first-principles based findings are consistent with recent experiment. The primary origin of the exotic AIE phenomena is due to the nonradiative decay effects. This is the first time that AIE is understood based on theoretical chemistry calculations for aggregates, which helps to resolve the present disputes over the mechanism.