Pocket PC Phone with Windows Mobile 5 Keeps Shutting Off

This doesn't happen enough to make me want to switch phones, but just enough to be annoying.  When I put it back on the cradle a lot of times, it turns the phone off.  Sometimes, if its clipped on to my belt, I'll look down and see that the phone is off, but I'll have no idea why.

Is there a "turn the phone off randomly" setting I'm not aware of?