Quite by accident, I actually got to watch this happen in a single player Planetary Assault mission today. I have come to the conclusion that this yet another variant of the spontaneously combusting wimgman/enemy bug. The reason that I believe this is that the planet went translucent at the moment that I destroyed the last defending ship, and proceeded to fade out as the break model pieces of that defender did the same. Somewhere lurking in the source code must be a small glitch that occasionally links two objects as far as damage/destuction goes. It is hard to figure out why, because it doesn't always happen. But, I would venture to guess that it might have something to do with the distance between the two objects that become linked. From now on, I will pay closer attention to where things are in relation to one another, and see if that has any effect on the occurrence of this bug.