Meanwhile, radiation from the sun is heating the reflective film. It, in
turn, heats the innermost part of the inner pane.

Inner pane is now hot on one side, cold on the other.

Bang.

I agree this is likely what happened, but why did it happen today and not
any other with the last two months? Today was a perfectly sunny day, and in
the low 30s. There have been perfectly sunny days much colder (teens) and
also there have been perfectcly sunny days much warmer (50s) and yet today
was the day this crack happened. During this time period (two months) the
sun was never obscured even at the very beginning of winter. Just wondering
why today?

J.