People who have woken up with screens cracked in the morning that weren't cracked the night before aren't necessarily out of the woods for blame either. A drop, flex, ding, bump, any manner of mishandling the day before could have resulted in a tiny, potentially microscopic fissure at one side of the glass. Then they put the phone on charge for the night, the battery heats up, the screen heats unevenly (as glass does when heated from one point rather than across the entire surface), and the variation in expansion of the glass at one end versus another adds stress to the fissure, causing it to release the stress by cracking.
If you've ever handled a glass cutter (carbide steel wheel on a spindle), you would know that in order to cut glass, you first stress the surface by scoring a line across it, then you flex it at the line to create the "run", cracking the glass along the scored stress line. Take that same piece of glass and just stress the edge, then place it in an similar situation to the phone, where you heat one area and not another...and watch as the glass cracks wildly in no distinct or obvious direction, but just takes off across the glass sheet.
I am not defending Motorola here, I am trying to level the playing field a bit. Blaming a manufacturer for making what the public has been demanding - a thinner phone, and then having it suffer cracks due to putting it through the same types of stress as thicker and less flexible phones is unfair.
Even Glass can bend and flex, even though it's extremely hard. I am sure you consider a Golf Ball as a pretty hard object. Watch this video and you'll think differently.