Most probably the pressure in the magma chamber reaches the point where the strength of the volcano and the weight of the volcano are no longer able to keep the pressure under control. Other folks have suggested that the tides can trigger eruptions. Most of the time when you think of tides you think of tides on the ocean caused by the gravitational pull of the moon and sun. Of course these Earth tides are much much smaller than the ocean tides, but they do exist, and they involve slight distortions of the Earth's surface. There were some suggestions that typhoon Yunia may have triggered the big Pinatubo explosion back in 1991.
Some folks think that the low atmospheric pressure was enough to trigger the big eruption.
That has not been proven, and it may never be proven.