A violent eruption usually means very vicious lava (rhyolitic). The viscosity of lava has to do with the amount of silica in the lava. The more silica in the lava, the more viscous (thick and sticky) it is. This is because viscous lava traps gases and allows them to build up more than thinner lavas allow. When the gases build up enough, the explode out along with the lava.
A violent explosion also means the expulsion of lots of pyroclastic material. Pyroclastic material is made up of pieces of molten rock (lave) that harden once they hit the air. These pieces range in size from huge chunks to hot ash.
Pyroclastic material moves extremely fast and is what causes the most damage in an eruption (pyroclastic flows can level entire forests). When this material mixes with flowing water, it creates fast-moving muddy rivers called lahars which can also cause a lot of damage. If people witnessed the eruption, they probably would have seen these things as opposed to slow lava flows