That wasn’t a window - it was an emergency escape door. What do you want to bet the passenger next to that door opened it ?
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Emergency exit. You are correct Sir.
I expect not secured/reinstalled properly. That exit should not open during pressurized flight. It has to come in first before opening unless design rules have been changed, same as the normal passenger doors.
I know an ex Alaska maintenance guy, he said he would bet $100 it was emergency door that was not secured properly by the flight crew and after investigation all flight crew will have to take a refresher course on how to properly close a door.