Lightning strikes and humans are probably the most common causes for fires in the desert.

Most desert areas in the US are a lot more lush than the blank sand dunes many people think of as desert. Usually there are a lot of bushes and grasses, and higher elevations can pine, juniper, oak, and other trees.

But PJ high desert areas don’t have the same level of destructive wildfires as, say, the Ponderosa pines found at higher elevations or cottonwood bosques. My impression is the fire load per acre in PJ is much lower and the fires are just not destructive. Adobe/masonry construction probably helps too, as does lower density urbanization. Probably easier to fight such fires too. (Limited experience)