Mostly the latter of your two suggestions. Franck Condon factor is also enhanced due to larger change in nuclear coordinates in the dimer vs monomer (if you will, the dimer explores a larger conformational space due to less restricted motion), and in these kinds of systems intersystem crossing efficiencies usually play a role as well. Which effect dominates in the control of nonradiative decay rates is probably a complicated function of solvent polarity/viscosity, temperature, and pyrene concentration.