I will gladly read any other theories, but here is mine.
When the gun recoils, the cylinder stop tries to stay where it is (courtesy of Mr. Newton), so it pivots and jumps out of the cut in the cylinder for the chamber that was just fired. The recoil also twists the gun and the cylinder turns just enough that the stop does not drop back in the slot. This means the cylinder is free to turn.
On a S&W, when the trigger is released, the hand is retracted. The cylinder can then turn backwards and the weight of the unfired rounds on that side (shooter's right) brings the cylinder back part way to the previous chamber. When the hammer is cocked, the hand rotates the cylinder so that the same chamber, the one just fired, comes under the hammer again. Click.
I am not sure why this happens mainly on the sandbags, but recoil does very funny things and I am sure it has something to do with the fact that the gun is not free to recoil as it would be if hand held.
One possible solution would be to try a stronger cylinder stop spring. (Did you, by any chance, replace any springs in the gun or have work done on it?) You might also try a different method of resting the gun. Resting the barrel or butt is not usually a good idea because the results obtained will be far off those obtained if/when you shoot the gun off hand.
Jim
HTH
Jim