After years of working with patients and their broken glasses, my experience has been:

• Glasses were sat on (usually when put down on a bed while dressing/undressing), or stepped on.
• Glasses were damaged during a sports or recreational activity.
• Glasses were “altered” by a family pet…almost always a dog.
• Glasses were destroyed by a young child. Could be the child’s or the parent’s glasses.
• Glasses were damaged in luggage, backpack,etc.
• Glasses were damaged by owner while attempting to do a repair at home.
• Glasses were damaged by “unknown entity.” Were found destroyed on nightstand upon wakening. “They must have been defective.”
• 

Please note that I am not counting loose or missing screws, broken nylon cords for rimless. missing nose pads or temple tips. Those parts are easily fixed and do not constitute damage. Also, home repairs using super glue, toothpicks, standard solder, epoxy resin, acetone, etc. will elevate a low level repair to the need for frame replacement, and will void manufacturers warranties.