I had something like this happen to my '91. I was at a friend's house when a thunderstorm came through. There was kind of a downburst event, and we all looked out the window to see what was going on outside. I saw a bunch of what I thought was just gravel suddenly blow down the street.
Later, when I went out, the entire rear window was gone. But I was surprised at how little glass wound up inside the vehicle. What we'd seen blowing down the street was not gravel, but my window! It was kind of odd because the wind direction should have blown the glass into the Explorer.
The best I can figure, the very high winds blew a rock down the street which struck the window and caused it to shatter. I did find a rock about an inch across in the vehicle that may have been the culprit. That storm did some damage around the area. Not a tornado, but just a downburst/microburst or whatever they call it.
Tempered glass like these back windows virtually explodes if it happens to get hit just right by something very sharp and hard. They make little hammers with a sharp carbide tip for breaking out side windows so you can escape in the event of an accident or finding yourself trapped in the car underwater, etc. They're incredibly effective with very little force.
A locksmith that came to open a friend's car one time showed us how he got it open as he was doing it. He used a bent wire device to lift up on the latch mechanism inside of the door. But he warned that if you so much as let the tool touch the bottom edge of the window glass, it could explode that way due to any bit of fine grit on the edge of the glass being pushed against it. He said it happened frequently enough that his business liability insurance covered it, but wanted to warn us that it was a possibility. Luckily it didn't happen that time, but it brings home that same point about how these tempered glass panels have a lot of stress in them, and can explode into a million tiny bits when so much as touched wrong with anything hard and sharp (like a particle of quartz or the like), particularly if touched on the edge of the glass.
I'm just throwing that all out there. It's one possible explanation for some of these events.
In the case of mine, I think replacement of that back window ran me about $90. They had a lot of these back glass pieces to fit that year of Explorer sitting around their shop gathering dust, and gave me a good deal on it because they were kind of happy to get rid of one.
