An underlying reason is the nature of the material. For example, a thixotropic material such as mud or sand and water, can change from a gel to a sol by adding water or pressure, or shaking. It goes from being stable one minute to fluid the next minute. The greatest disasters, like lahars or mudslides, happen suddenly. It is possible for much of a volcano to just slide down suddenly, as with Toba, Krakatoa and Mount St Helens.