Mudslides are a hazard caused by heavy rains. There are layers of soil under our feet. The further down a layer of soil is, the more compressed and stable it becomes. When the topsoil becomes saturated with rain, it becomes heavier. This, combined with the flowing water, can lead to that top layer of soil sliding downhill.
There is a similar process for debris flows. As vegetation burns in a fire, it can leave behind a water-resistant layer just below the surface. As heavy rain falls, the topsoil becomes saturated, just like in a mudslide. Once the topsoil becomes saturated, it can easily slide off the water-resistant layer, leading to a debris flow.