.. The manufacturer
recommended against putting it on the inside of double-pane windows, but I
tried it anyway,

Therein lies the problem.
*follow manufacturers' recommendations*

Reflective film reflects a large amount of radiant heat directly into the
hollow between the windows, overheating both panes. That is why all film
manufacturers are loathe to put them on double glazing. Likely as not
simple heat caused thermal crack; not necessarily expansion/explosion of the
center gas, as you suspect. The added insulated shade may have exacerbated
the heating effect, especially on the inner pane. So you had trapped heat
heating both panes, and the shade adding extra heat to the inner pane.