It seems to me that if there was a problem with the trailer that caused the window to break, then both layers of window would have broken. While it's very unusual for something to hit the rear of the coach while going forwards, it's certainly not impossible. And it is likely that something striking the rear of the coach while you're going forwards would have a limited amount of energy, so breaking only one layer of the glass would be a possible scenario.
I'd suggest contacting your insurance carrier ASAP and filing a claim under your comprehensive.
Steve & C. J.
2009 Arctic Fox 29V PullRite 20K hitch
2005 Chevy Silverado 2500HD 4X4 Duramax/Allison