In March 1991 and April 1991, magma rising 20 miles beneath the surface of the Pinatubo volcano caused many earthquakes and small explosions, which destroyed parts of the volcano. Then on June 12, 1991, magma filled with gas reached the surface, and the volcano erupted.
Before the eruption in 1991, the Volcano had been largely inactive for 500 years. On July 16, 1990, there was a earthquake of magnitude 7.8 that occurred 60 miles from the volcano that disturbed the Earth’s crust beneath Pinatubo. Although this earthquake caused smaller earthquakes and landslides near the volcano, it did not appear as though these events had significantly agitated the volcano.
The first magma that started rising to the surface in March 1991 lost most of its gas content on the way up. So, when it reached the surface, it just slowly oozed out and fell down the volcano's slopes. After the initial explosion, there was more magma on June 15, 1991, that caused more violent explosions due to it having a much higher gas content.
The gases in magma normally dissolve when they are deep beneath the surface. However, these gases, which were responsible for the eruption, expanded as the magma rose because of the low pressure.