Any reason why sealed unit would crack on its own???

Hi, we fitted 2 new double glazed, top opener windows to our bedroom saturday, fitted no prob - looking great. Went to bed sunday night to be woken by a scratching sort of sound coming from window, when i checked to see what was happening 2 cracks had appeared both starting from the same point at the bottom of the window travelling across / up in a "V" shape with small splinters of glass inside unit, the crack is on the inside and a ridge is evident when touched. Does anyone have any idea why this could have happened when it was fine for over a day after fitting and cleaning?? The glass is pilkington K.
Thanks
Leanne

The packing piece or the unit not packed/centred properly? or something on the unit seal?

May also have been an inherent flaw or slight crack which got worse after fitting in the room and glass expanded/contracted

it sounds like the edge of the unit is caught on a packer or part of frame.i have had this happen when unit has been in for months.try placing packers at right angles to frame,put new unit in and twist packers so they are level with the edge of the unit closest to the bead -dont push them in past level or the unit may catch again.