What's causing iPod touch screen to break spontaneously?

I was wondering if there is information out there concerning spontaneous breakage of iPod Touch screens. Both iPods I gave my kids have suffered broken screens, one occurred a few seconds after the charger cable was unplugged. The other happened shortly after the unit was placed on a docking station. It doesn't make sense and it seems like the screens are under some tension/stress and break without dropping or other trauma. I looked around the web but didn't immediately find anything on this, or the possibility of manufacturing or design flaws. Is this a common problem and how can it be avoided?