Physical mechanisms are discussed and crude mathematical models with lumped parameters are developed, which explain the authors recent experimental data [4], concerning temperature hysteresis and multiplicity phenomena for alpha-methylstyrene (AMS) liquid-vapor hydrogenation on a single catalyst pellet. The interplay between endothermic vaporization and exothermic vapor phase reaction is elucidated. The results of this study may help to develop more sophisticated models and theory of hot spots formation and runaway phenomena in trickle-bed reactors.