In this paper, a mechanism is proposed for the occurrence of "early" tsunami waves on various coasts of the Pacific Ocean, which arrived after the eruption of the Tonga volcano 4–5 hours earlier than the actual time of the tsunami wave arrival at a given sea level. The paper shows that Kthere is another alternative hypothesis about the generation of an early wave train by a pre-eruption landslide source, which is localized on the slope of the Tonga volcano, in addition to the hypothesis available in the literature explaining this phenomenon as the generation of tsunami waves by Lamb waves or caldera collapse. At the same time, during the period of preparing for the eruption, the trigger for the pre-eruption landslide process could have been both minor seismic foreshocks, which were numerous before the eruption, and the process of preparing for the volcanic eruption itself. Wave characteristics of the tsunami generated by the pre-eruption landslide process on the volcanic slope are obtained. A possible implementation of the pre-eruption landslide mechanism for generating "early" tsunami waves is presented by considering various scenarios of pre-eruption landslide source formation and taking the tsunami characteristics at specific sites into account.