Volcanoes like Soufrière Hills produce lots of volcanic ash when they erupt. That’s why the summits of many volcanoes are covered by a thick layer of ash. Rainfall can remobilise this ash to form so-called mudflows or lahars. This are mixtures of water, ash and other volcanic debris, running down the volcanoes following river channels and destroying everything in their path.
Lahars can occur years after volcanoes stopped erupting, making them an especially dangerous hazard. In Montserrat, heavy rainfall is common, and so are lahars. The video below shows a lahar in Belham Valley in 2016.