This thread predates me, but now that it was bumped I will chime in...
I am an automation and motion specialist (engineer). I work with LOTS of companies in LOTS of industries and assist with both the front end engineering on automating a process as well as back end (make it work) assistance. This gives me a lot of insight to how things are made and why. You can't automate a process you don't understand.... I have three large auto-glass manufacturers as customers.
Automotive glass is tempered. Tempering not only makes the glass harder but also has major safety implications. Unfortunately, Tempering also makes the glass brittle (especially on the edges). The smoothing or grinding of the edges are not just for aesthetics or installation safety (it's actually installed by a robot). It's there to help deflect things that could chip the edge. Tempered glass is always under stress (both compression and tension), so when it's chipped or even nicked that energy can be released and the result is a a total shatter into a bunch of pieces.
Now, understand not every piece of glass in your car is the same, each are made differently. Front windows have a laminated piece of plastic in them. This is there to keep a stone chip from shattering the glass so badly that you can't see out of it and to keep it from falling in on you. Side and rear windows don't normally have this lamination. There are lots of good reasons for this.
The bottom line is that without this lamination, even the smallest of nicks can cause the glass to "explode" or shatter.
It looks to me that the OP's glass was being squeezed from the sides and finally got to the breaking/shattering point. But to some of the others that commented, lots could be going on.... Since it only takes the smallest of nicks to start a chain reaction it is really impossible to know if you were not a victim. It's a long story, but I grew up in a shady area... I know guys that carry spring loaded center punches or broken spark-plug ceramic pieces. Throwing these pieces at a window, or using the center-punch leaves an undetectable source, but will absolutely do this to a window. (I actually carry a center-punch in our cars for the fear of ending up in water and being trapped.) A BB or pellet gun will also do this and oftentimes glance away again being undetectable.
Another possibility is just thermal stress. This is pretty common on old cars. My oldest son's car had this happen to him. He was walking up to it after track practice and he saw his glass suddenly shatter. I wouldn't expect this to be an issue on such a new vehicle, but I can tell you that mfg's of glass are being pushed for harder and harder glass, The shape of the glass also has a big part and opens the opportunity for manufacturing defects.
Anyway... This issue is both common and rare at the same time.... I wouldn't stress it, but know it can happen. If you have little ones in the back you might consider window tent or a second screen to keep any glass from hitting them. While the chances are slim, it's pretty cheap insurance to keep your loved ones safe. NOTE: depending on how the factory glass is tented, it may or may not have film in it for the tent.
Hope this helps.
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