That wave of water that forms in front of the tire....make that a wave or roll of sand!
The front tire must climb over that wave/roll of sand. (Which gets bigger and bigger as you move foreward)
If tire can't climb over the roll of sand, it begins to act like a block and your forward momentum slows or stops, then the rear wheels, because they are light, begin to bounce, digging a nice hole in the sand!
AND! Down you go!
How's that sound? <img src="confused.gif" border="0">