For a long time – centuries, really – people have noticed that sizable rocks (the so-called “sailing stones“) sometimes slide across Death Valley leaving long “racetracks” in their wake. But nobody had actually seen this happen. It was a mystery as to just what was going on… theories included the rocks being shoved by wind after the playa had been rained on and, of course, ghosts and spirits and aliens and all the other nonsense that people like to invoke for anything not immediately explicable.
A series of tests involving rocks with built-in GPS locators and long-duration fixed cameras have finally solved the mystery.
At rare times in winter, shallow ponds will form. The dirt and salt and whatnot that gets dissolved into the occasional rainwater doesn’t flow away, but is simply left behind when the water evaporates; this results in a very flat, uniform “floor.” When a pond forms, the water will be broad but very shallow. And if it’s cold enough, it will of course freeze on the surface. During a thaw, the ice will break up, first into large sheets. These sheets will be thin, just a few millimeters, but very broad, and the wind can use the low, broad sheets as sails to push on any rocks projecting above the dirt.
Nothing magical or supernatural or even that conceptually difficult. It’s just that since the population density is so low, apparently nobody was actually there to see it before until recently.