Many more UK bikes develop that problem than any other country I've seen. I'm pretty sure it's because of riding on wet, salted streets. As I see it, the salt water seeps into the wiring harness, corroding the crimp connections inside the loom. The corrosion causes poor conductivity, creating heat. The heat softens the insulation, allowing the wires to contact the frame. That creates even more heat until the area over the frame gets cooked and shorts out.
It's often attributed to a rough or sharp area on the frame but it's rare in places without salt water and I never found a rough or sharp spot on my bike. The pictures of damage I've seen look like widespread heat damage rather than cutting.