Interesting observation, Frank. I've kept Westerns for 8 years, and have never seen this behavior. I witnessed it in the first Mexican hog we came across in AZ.
The advice we were given from a friend who'd had a great deal of success observing them in the wild was that there was a temp related window for finding them (ground temp between 81 - 90F. Though we didn't have a temp gun, the air temp had been @ 78F for good while. We parked our car, and as we walked up to the snake it flipped over, doing its death-feigning display. The smaller female we encountered later, however, did not do this.

reako45