I do not know for sure how it came off. I was out in the woods at the time, realized the battery door was not on the camera, and found it nearby on the ground. In order for it to fall off, sequentially the door had to unlatch and open first. Then, once it was hanging loose, get brushed against something that dislodged it from the camera. It could have been my error to securely latch it the last time I replaced the battery, or perhaps I brushed the bottom of the camera against some brush or something that snagged the latch and opened the door and I didn't see it. I really do not know - it only happened once.
Although clearly it is a nice feature that I wouldn't change, the battery latch keeps the battery in place allowing the camera to keep working if the door accidentally opens. Once its open, and hanging by the hinge, it is easily dislodged from that point. If the battery fell out when the door opened, then you would know it immediately if you were actively shooting because you would loose power. But I like the D800 battery latch the way it is. Just recognize that if the door is accidentally opened,and you don't notice it, it can then be easily kocked off hanging in the open position.
For me, there is a simple fix using a small piece of electrical tape as a safety measure.