I've found that if I set it down on it's end (as in on the battery cap) with more than a certain amount of force, the battery, having the mass/inertia that it has will compress the spring in the battery cap sufficiently as to cause it to break contact between the top of the battery and the top contact...hence the powering off.
Not talking slamming it down, but more along the lines of letting it drop an inch onto the ba
throom sink flat surface area (seems where I do it the most..I think my wooden coffee table absorbs some of the shock). That lil drop is when I will find a minute or two later that it has shut off.