Holy crap that sucks. I had the rear window shatter on my 1990 Olds randomly on the highway. Never found what hit it but I'm assuming it was either a part of the road (because New England) or a piece of concrete from the tunnel (because Big Dig).
This was in January, in a snow storm, in Boston while it was snowing. 4 hours to get home, I'm so happy that boat had good heat.
I don't know why they don't make them out of safety glass like the fronts...