They are susceptible primarily to corners, and after that, to edges. The sharper, the worse. The way they are made, and the type of aluminium they are made out

They are susceptible primarily to corners, and after that, to edges. The sharper, the worse. The way they are made, and the type of aluminium they are made out of, results in them failing after experiencing comparatively little strain (for being a ductile metal alloy). Because the walls are so thin, squeezing them with the grip of a hand won't cause much strain, due to the large bending radii (seen relative to the wall thickness). But if you concentrate the force around a (comparatively to your fingers) sharp corner/edge, especially if it's a corner sharp enough to hurt if you pressed on it with your hand and, say, ~5kg worth of force, you'll force it to stretch to conform to the pointy corner's shape. (Despite the factory already pushing the material close to it's limit as far as stretching-before-ripping goes. No wonder it rips, then.)