One of the most important and widely used renewable energy sources is hydroelectric energy produced via Water Reservoir Impoundment (WRI). WRI can trigger strong earthquakes under favourable geological conditions. Thus, the socio-economic impact of reservoir triggered seismicity is very significant. Although many studies have investigated the relationship between the pore pressure changes due to WRI and the observed seismicity, hydromechanical models that explain the observed processes are rare. Here, we investigate the role of hydromechanical interactions during fault deformation to understand earthquake swarm bursts under pore pressure changes due to WRI. As a natural laboratory, we selected the Song Tranh 2 Reservoir in Vietnam. Because the analysed triggered seismicity has swarm characteristics, our work contributes to the further investigation of the physical mechanisms responsible for earthquake swarms and their relationship to slow slip. We conclude that the small high-frequency seismic swarms accompanying WRI are driven by slow slip along a fault; they occur due to the temperature-controlled frictional fault heterogeneity, and their rate and magnitude depend on the sizes of these heterogeneities. Swarm earthquakes are the effect of slip acceleration on the seismic radiation level. The nucleation fronts expand the nucleation regime and may transition into stronger earthquakes. These results provide insights into the physical mechanisms of seismic processes triggered by WRI, which may have implications for assessing the seismic hazards associated with hydroelectric energy production.