My guess is mechanical stress during the print.

Think of your print object as a lever. The attachment to the print bed is the fulcrum. The taller the object gets, the longer the lever arm and the more potential for movement, especially while the plastic is still warm and soft.

On the other end of the lever is the nozzle spitting out melted plastic. The melted plastic is sticky (PETG in particular is kind of like chewing gum at print temperature). As the nozzle moves across the printed surface, the sticky plastic pulls on the previous layer, exerting a lateral force (you can watch this happen during the print, it’s most obvious with tall thin parts). If there isn’t enough contact area between the topmost layer and the one below it (which in your case it appears those parts of the hexagons have very little contact with the layer below) then the top layer can be ripped off.

Basically the individual limbs of the hexagons are too thin, and the angles are too steep. As the print gets taller the whole thing will flex more, making failures more likely near the top.