Not sure of how snake's bowels work but heck if you were passing something that huge compared to your body size maybe it's just a matter of "omg here it comesssssssssssssssss" and they don't have the control to stop it and move elsewhere to evacuate their bowels. Most of ours do seem to poop in their hides though and then move to the opposite hide, just seems to be the way they take care of business lol.
~~Jo~~