Freezer Door Glass Cracked

I have a tripple glassed (freezer door) that I brought to put in as a glazed house window, hoping to combat loosing heat in winter. It measures 1500 high X 485 wide. It has 240V wiring hooked up on the long side.

I placed the door in the sun for a month to see how it would stand up to the elements of direct sunshine. The centre plane of glass has cracked, but not the outside glass pieces. This is strange.

Can anyone who is in the know suggest why the centre pane of glass has cracked and not the outsides panes? I believe the door maybe vacumn sealed.

Thanks