Back windows have shattered on the Edge from time to time, have not heard of it on the X yet tho. On the most recent MYs for the Edge, they blame the rear defrost heater connection and have a CSP 17B39 out to replace the glass if it does break.
"In Canada they put a lot of salt on road and the melting snow or ice with the salt can enter in the rear wiper motor and it become very hot
The rear window is at external temperature below zero and the motor is hot result that the rear glass broke with the difference of temperature between both sides
The repair is to remove the rear wiper motor, clean it and the axle to remove all trace of salt, install a new bushing in the rear window with silicon grease reinstall the motor and with the silicon grease and the new bushing the salt and water will no longer enter in the motor"