Having been involved in bodging the old style plug-in supplies for the two G4 ibooks in the house I can tell you that it is due to rough and negligent handling. The angles I have seen the cable stretched with no thought beggar belief only in the sense that is remarkable these things don't fail more often.

Watch for the followup suit where the same people discover that the way they have abused their power supply has had consequences for their batteries. It's amazing, how much do these cost and then people treat them like they are disposable.