Why did this gas lift assist unit fail this way?
The image is the result of an out-of-column force being applied to the mechanism. In the case of the gas lift device, either end of the attachment could have something in contact preventing the cylinder from following the expected angle change as the cover is raised or lowered.

Not to say that this is the specific cause, but if the tub end of the cylinder has a buckle hanging over the edge of the tub and it gets wedged into the area of the attach point for the cylinder, it could impede rotation of the cylinder and create the aforementioned out-of-column condition.