With the extreme heat in our area and with the extremely high fire danger I felt it was my duty to warn you about this seemingly harmless product that could ruin your life...

The product?? Mulch particularly dark colored mulch. Here's a post I came across and the damage it caused

After doing some research on this topic here's what I found:

Spontaneous combustion occurs when a substance generates enough heat to ignite without an outside source. ... Since mulch is a wood product and is always decomposing, a large enough pile of it can build up enough heat and spontaneously combust

Mulch is made of wood and as the wood begins to decompose it produces heat. When mulch becomes too dry, the heat can build until a fire begins from spontaneous combustion. Dark mulch can heat more rapidly than lighter colored mulches. Mulch catches on fire because the sun dries it out