I saw a show a short while ago on the discovery channel or similar that did a good deal of testing on rock trails on the flats.

They actually took a good amount of soil from the surface of death valley, as well as several rocks which exhibited trails, and placed them all in a wind tunnel.

They simulated the usual wind levels the valley sees and concluded that on dry soil, the rocks won't really move. They moistened the soil slightly though, mimicking the brief rains they get on rare occasions, and the rocks made identical tracks through the soil, even replicating the erratic paths often seen. The amorphous shapes directed the rocks in different directions in a sustained wind from one direction...which I thought was the most interesting part.

Mystery solved!