National Geographic speculates that Mt. Fuji is developing a large reserve of basaltic magma based on an increase in earthquakes first noted in the early 2000s. The heat and gas brought to the surface by basaltic magma causes these quakes.
The destructive force of the 1707 eruption may have altered the internal structure of the volcano, meaning that it may currently contain a different type of magma than it contained in the past, when the volcano erupted somewhat regularly every 30 years. The Japanese government fears that the current dormancy of the mountain may be allowing it to stock up on a more explosive type of magma.