Another possibility is the manufacturing tolerances of the mounting. There could be stress placed on the glass due to the mounting tolerances being at extreme ends, e.g. the upper left mount point at positive extreme and the lower right being at negative extreme. This condition would be rare in the manufacturing process but the glass would be under constant stress such that a slight knock or even temperature change could cause the break.
Tommy is correct in that the break seems "spontaneous" (i.e. no overt cause for the break) and in fact is not the fault of the user. However the statistical odds are for the user causing the break so the manufacture would accept that as the manner of breakage.
As Harry says this would be extremely rare. But that doesn't help the person who experiences it. Unfortunately we don't hear about the hundreds, thousands, or hundred of thousands of units that don't break in this manner, so it
seems like it happens too much for it not to be a manufacturing fault.
