On the early Comet series, it was not so much the square windows but a combination of a brittle alloy and slightly inadequate manufacturing practices. Many openings were not drilled or cut but rather punched, leading to microscopic cracks around the opening. Also, despite the plans calling for Redux glue in this area, rivets were installed for some reason that also had their holes punched instead of drilled.
With G-ALYP, the investigators
seem to have formed the opinion that the destruction of the fuselage has consequently started not on the windows but on the mentioned ADF hatch. On the subsequent ALYY accident, it seems that insufficient wreckage has been found for a similar analysis but the board had concluded from the similarity of the surrounding circumstances that the destruction of the aircraft occurred in a similar way.