The observed anti-Arrhenius temperature dependence of the rate for positronium-acceptor (Ps-Ac) complex formation above, and a normal Arrhenius behaviour below, a solvent-dependent critical temperature T-0, is argued to be ascribable to a Kramers' turnover, relating to viscosity effects unleashed by temperature-dependent surface tension, in the context of the bubble model of the positronium atom in liquids, provided one uses a more realistic and robust version thereof developed by us, which is also in closer accord with lifetime and gamma-gamma angular correlation data in pure solvents.