Rob,
I have found that a number of my WO lens cells, namely the FLT132, FLT110 and Z66, came loose over a period of time because the dew shields may be slightly tight when sliding the shield in or out. My usual action is to hold on to the tube and use a twisting motion on the shield to move it in or out. If you just pull on the shield you feel that you'll pull the scope over.
If, over a period of time, you turn the shield anticlockwise then the cell may undo very slightly each time. I now turn or twist the shield clockwise to overcome this problem.
Some manufacturers use a small grub screw to stop the lens cells from unscrewing however to the unwary, a lot of damage can be done to the tube threads and cell threads if you try to remove it.
You WO guys and gals out there may wish to see if your lens cells are loose also
Cheers,
Steve