As stated above, I'm sure deterioration from misuse on the load side is gonna be right up there.
Also, I would guess that, if the homes were old enough, they probably found quite a few exposed wire joints. I've found that the old solder-and-wrap method of wire joining, although good from an electrical conduction standpoint, suffer from deterioration of the wrapping materials and don't handle maintenance and handling too well. Also, a few times being pulled in and out of the junction box for add-ons and repairs tends to break up the old cloth insulation pretty badly too.
So, I would guess that they would classify this as "post-build maintenance damage", or something like that.
Induced voltage is also prevalent in these old installations.
Improperly bonded and altogether loss of the grounding system also comes to mind.
