Those big old axle nuts do loosen with time. I have to check mine once in a while, but my bike shakes and rattles more than a Commando.
If you get the speedo shim setup right, you can crank the axle down tight without worry.
I made a mistake shimming a new speedometer drive unit when I first put a speedo on my Norton. Rear wheel locked up. Cable didn't wrap around the axle, but it snapped at the speedo and was dragging behind the bike. The speedo housing got jammed up on the hub drive, and stopped the wheel from spinning. Your axle must have been a little loose at some point, or the speedo housing would/could have stopped spinning around wrapping up the cable and instead locked up the rear wheel. Well, maybe anyway.