The seismic activity mentioned in the question is due to subduction at the Sunda Trench, where the oceanic plate is diving beneath a continental plate, causing earthquakes and creating a line of stratovolcanoes.
The process occurring along the trench when an oceanic plate dives below a continental plate is known as subduction. This process is taking place along the Sunda Trench, which is responsible for the creation of a line of stratovolcanoes, including Tambora and Krakatoa, as well as regular earthquakes. When subduction occurs, the denser oceanic plate is forced beneath the continental plate into the mantle, leading to high pressure and temperature conditions that can melt the subducted materials, forming magma that feeds volcanoes.
Earthquakes result from the movements of these plates and the release of tension accumulating in impacted rocks. This cycle of plate movement and interaction, with new crust being created and old crust being destroyed, is a fundamental aspect of plate tectonics.
So, the seismic activity in the region described is a direct consequence of the subduction of an oceanic plate beneath a continental plate at a convergent boundary, which also contributes to the volcanic activity observed along the Sunda Trench.