It would be possible to just be gnawed at the one place, but where I've seen gnawing before it's spread around a bit, at least like a half an inch or so. And I'd be really surprised if anything like that could even reach it - but I've been really surprised before. A nearby lightning strike is a real possibility, though if this cable was the ground path lots more would be toasted, like probably the entire system? My best guess at this point is a kink or other stress (pulling too hard?) during installation that caused minor damage, but enough to increase (no short) or decrease (short) resistance at that point.
Also, with 3 amps available across 4 equal power wires, if one of them had less resistance due to damage or whatever, then up to almost 3 amps could be available(?) to that one damaged point, maybe allowing this kind of toasting? How much current is available to one wire, at a dead short on that one wire, with the power supply you mentioned before the breaker pops? I'm assuming that there is a breaker for each lead, or I think the available current could be lots higher?
With that description of siamese cable, it does match my thought of something like O8 or Oo shape. This one is a coax, just one O shape inside one molded jacket. This one is about 3/16" cable diameter, and is "factory split" at each end into the video connector and a power plug type connector for the 12v. All the splits and connections are molded and insulated, like this one -
I have seen some with only about 1/8" diameter. These are also cheaper. These just seemed to me to be... cheaper, less well made, though I might be wrong.
Thanks!