Tempered glass is subject to spontaneous shattering. It is also possible there was something in the channel that pressed on or scratched the edge. Any edge damage on tempered glass makes it shatter.
Bummer. Hope you can replace it.
Sent from my pocket Internet using Airstream Forums
'The Silver HamShack' (2007 International 22FB CCD 75th Aniversary model)
Multiple Yaesu Radios inside and many antennae sprouting from roof, ProPride hitch
2012 shortbed crewcab 4x4 Toyota Taco TV with more antennae on it