Question: Why are some eruptions so explosive ?

Excellent question!

the explosiveness of a volcano is dependent on how much water and gas are trapped inside the volcano. In Hawaii for example you get magma that has almost no water and no gasses trapped inside and so you just get really boring lava eruptions! but in places where there is subduction you can end up with lots of water and gasses trapped inside your magma and this makes your magma really sticky a bit like treacle or molasses and this magma gets stuck inside the volcano. When the magma finally reaches the surface and the eruption begins, the amount of pressure on the magma becomes smaller and all the water and gas in the magma tries to escape. This is a bit like when you open a bottle of soda and you hear all the gas fizz out of your drink! the volcano does exactly the same thing! But as your magma releases all of it’s gas, the gas expands and rips apart the surrounding magma into tiny pieces. This happens over and over again and really really fast so that you end up with these tiny fragments of ash instead of a nice orange lava flow!