How did this natural disaster happen?

In the early hours of 10th June 1886, after a series of constant quakes since midnight, a violent eruption occurred near the township of Rotorua. The top of nearby Mount Wahanga was completely blown off, producing a dense black cloud extending from Taheke to Paeroa, and which became a mass of thunder and lightening to continue incessantly the night long. Shortly after, nearby Mount Tarawera and its twin cone Ruawahia exploded into action, belching fire. The process of a volcanic eruption is when the pressure on a magma chamber deep below the Earth’s surface begins to build up. This building pressure causes the magma from the chamber to be forced upward through a series of tubes or conduits. As the magma reaches the surface, it is thrust through the vents on the volcano, resulting in an eruption.