The fluorescence of most organic chromophore is emitted from the pi pi* state, whereas the n pi* state, as a dark state, plays an important role in quenching the fluorescence when its energy is close to the pi pi* state. Herein, we report a theoretical study on the fluorescence quenching of 8-methoxy-4-methyl-2H-benzo[g]chromen-2-one by the n pi* state and propose a new mechanism for describing the vibronic coupling between the pi pi* and n pi* states. By applying extended multistate complete-active-space second-order perturbation theory (XMS-CASPT2) to optimize the geometries, the geometry distortion of the ire state along the out-of-plane mode is observed. This geometry distortion causes the stretching vibration of the carbonyl group to be coupled with the C-C bonds of the pyran ring, which become a Franck-Condon active mode upon photoexcitation and provides a driving force for nonradiative decay from the n pi state, even if it is energetically unfavorable. This mechanism is significantly different from the previously proposed "proximity effect" and cannot be captured by the popularly used time-dependent density functional theory (TDDFT) and complete-active-space self-consistent field (CASSCF) methods.