First let me give the story behind this question.
I had this asked of me one time while trying to help some vacationeers find their way back to ther rental house during a Nor'easter. They had been following me for about a mile when I lost them going over a dune. I didn't want to drive back for fear that we would collide at the top so I had to get out and run. When I got to the other side I saw that they were trying to make it up, but would keep on bogging down about halfway. When they saw me, the driver rollded down her window and said "I can't get enough momentum". I looked at her tires and asked if she had droped their pressure. She said yes. I asked "what did you drop them down to?" and she repied "Thirty". I told her back it down to the bottom of the dune and let's drop them to fifteen. And during the depressurizing, I was asked the question.
As it turns out there is some truth to that possibility. This is an exerpt that I found on a couple of web sites.
"While it's common to lower tire pressure if you're going offroad, if you have 1/2" in the description of your wheel (i.e. 16.5x8 -- found on many newer SUVs), DO NOT reduce your tire pressure when offroad! The bead found on the 1/2" variety wheels is not deep enough to keep the tire on during 4-wheeling excursions if the pressure is lowered."
After having found this informantion I checked around on one of the 4X4 forums that I subscribe to. they all agreed that although this is possible. It would happen unless the pressure were dropped to single digits and running at 15lbs on the sand is safe but recomended.