While turning a corner [a left] my back driver's side window shattered and filled the back seat with bits of glass. As near as I can tell it all fell in. I can not see a rock amongst the debris. A BB or pellet may be there as they are small and I have not cleaned up the entire mess yet. Barring a misbehaving child, is there something in a power window system that can under rare conditions cause a window to shatter?
If there was a weakness or chip, the slight bending placed on a pane of glass by turning could be enough to shatter it. When you turn a corner in any car there is flex in the chassis - usually this wouldn't be enough to cause any problems though, as there is rubber holding the window in place and this should flex enough, so I can only assume something had already weakened the glass.
Of course the simpler alternative is a BB or pellet, as you mention - and one could even have done the damage and bounced so may not be discovered.